This statistic that "mana drain decks" are 45% of top 8s... What if I said, "In the last year, 45% of all vehicles in accidents [in a particular city] were green.  We can therefore conclude that drivers of green cars are much more likely than any other color to get into an accident.”
The flaw in this statement is that it doesn't list the control variable: what percentage of all cars in the city are green?  If 45% of the cars are green, then it would be perfectly natural that they have an equal representation in accidents.  If more than 45% of the cars are green, then it actually means there is an under representation of green cars in accidents.  If and only if less than 45% of the cars in the city are green does it mean an over representation of green cars in accidents, a skew in what the standard distribution SHOULD be.
 
Alright, back to magic.  What percentage of players are currently playing Mana Drain decks to contribute to this 45% top 8 number?  I would also go one step further and ask of the subset of all tournament players who are of the play skill level required to top 8, what percentage of THEIR decks feature Mana Drain?  Is it greater than or equal to 45%?  I realize you may not have this data, but the 45% statistic seems somewhat meaningless without an established baseline.  I don't see how this statistic, in and of itself either confirms or denies the balance of the Vntage metagame.  What I see is a format where the percentage of players using blue control decks has remained roughly the same.  The composition of those blue control decks is what has changed, shifting away from combo elements (that were restricted) towards control elements that remain unrestricted, the favorite choice being the combination of Force of Will and Mana Drain.  If you really want to compare the two metagames, what is the percentage of decks featuring force of will now compared to before the restrictions.  My guess would be that it's roughly the same.
 
Let's look at the tier 1 decks during the 2nd era of Gush:
Flash - no gush, pact over Drain
Tyrant Oath - gush, Mana Drain
GAT - gush, Mana Drain
Dredge
Workshops
and towards the end of that era, Painter - gush, Mana Drain
 
What was the percentage of people playing Gush in that time period and how close was it to the 25% representation in top 8s?  I really can't be sure.  But in my local metagame, when Flash, Tyrant Oath and GAT got neutered, the majority of players of those blue based decks decided to play another blue based control deck.  One other statistic is needed to complete this argument: what was the percentage of people playing mana drain decks WITHOUT Gush when Gush was restricted?  Those two pools of players have since combined to make up the total Mana Drain field as we see it now.  Fish, Control Slaver, Landstill, Drain Tendrils, etc are still around just like before, only now they're doing better and being thrown in with the “Mana Drain Decks”.  Going from 4 blue based control decks frequenting top 8s to anything more than 4 is increased diversity, not decreased diversity.
 
And really, is the increase in Mana Drain decks a surprise?  Something had to fill the void left by Gush and Flash, and Drain seems the logical choice.  It's unrestricted and a huge tempo boost, only defensive first and then offensive later where Flash and Gush were offensive immediately; they forced you to act immediately or lose.  The GAT lists were all basically a choice between the number of Duresses vs. Drains you wanted to run with your 4-6 remaining slots, with duress winning out over drain towards the end.  You were playing 8 duress GAT towards the end if I remember correctly.  And when 12 cards got ripped from the deck (3 gush, 3 scroll, 3 brainstorm, 3 ponder) what else was going to fill those 12 slots and take over as the card to build decks around?  It had to be Drain, it was simply the best unrestricted option for tempo boost in the format, with at least break even card advantage.

I’ve already written too much for one post, so let me respond to just one more thing for now. 

Let me be clear.  I am not arguing about that specific 2007 engine which is as you say, dead forever.  What is relevant now is what is available in the card pool vs. 2003, when Gush was deemed to need restriction in the first place.

In the last 5 years there have been many printings which can, in theory, support a chain of gushes that form a critical mass with or without Yawgmoth's Will.  Taking a look at the 2003 lists, Isn’t GAT more powerful now than in 2003?  How many cards have been added to the card pool to offset the loss of 3 brainstorms and 1 merchant scroll?  Imperial Seal, Grim Tutor, Ponder, Gifts Ungiven, Strategic Planning, Sensei's divining top, Dark Confidant, etc…  (and others like Personal Tutor and Serum Visions that are 2nd rate but available).  Gush has been broken both times it was legal.  I have zero doubt that it will be broken a third time if unrestricted and I wanted to voice my opposition to your statement.  Dream Halls, Chrome Mox, Mox Diamond, Black Vise, Voltaic Key, Mind Twist; unrestricting those cards made sense.  But this one just doesn't.

Reference: http://www.starcitygames.com/magic/vintage/5613_Thats_Gush_BoysWhy_Gush_Needed_To_Be_Restricted.html

Hey Steve,

I wanted to say first that I like the direction your articles are headed these days.  I’m finding they’re helping me improve my game more and more.  Thanks for that.

I wanted to comment on what you said about Gush in your most recent article.
“There is no reason that Gush cannot be unrestricted. This deck would remain perfectly fair.”

I wasn’t really around the vintage scene the first time GAT became big, 2003 I believe it was, but I saw that the decklists did not feature 4 Merchant Scrolls.  It was 4 Brainstorm, 1 or 2 Scrolls and 4 Gushes.  I do remember 2007 when I started to hate playing in tournaments and eventually stopped altogether.   I heard a lot of people saying ‘until Flash is gone, I quit this format.’  It just wasn’t fun anymore, playing who drew the most broken hand this time or who won the dice roll.  And then what you called the ‘Vintage Apocalypse’ happened.

In my opinion, in the time since the restriction of those 5 blue spells, vintage has become fun again.  I’m once again having a great time playing in tournaments.  What’s more, I’m seeing a lot more new players on the vintage scene than I did back then.  And some that gave up on Vintage, like me, are returning to the tournament scene and finding it very enjoyable.

But let me talk specifically about Gush.  I like that Wasteland/Strip Mine is good again, now that it’s a 5/1 ratio as opposed to 5/4.  The autoresponse of Gush to a Wasteland or Strip Mine killed fish decks in that era and greatly weakened Workshop decks.
   “Like the Gifts Ungiven strategies that Tez decks resemble, Null Rod based strategies give this deck fits.”

While Tez may be putting up the best numbers, I like that the top deck has an inherent vulnerability to null rod + stifle + waste/strip.  Just last weekend I was able to use that strategy to beat Tez, Oath and MaskNought and get into the top 8 in a tournament in Montreal.

I also like that Mana Drain is back with vigor and decks are being built for consistency and play skill as opposed to a race to find Flash or Oath of Druids or Yawgmoth’s Will and go off.  I like that in my local metagame, Slaver, Tez, Dredge, Landstill, Dragon, Fish, Combo, Painter, Stax, Shop Agro, Zoo, Faeries (+the rest) and even people’s own creations are all viable decks seeing action in the top 8 of tournaments with large turnouts.

In my opinion, if 4 Gush comes back into the fold, a lot of those decks just become unviable and the diversity that has finally returned to Vintage will lessen or possibly vanish.  Going back to what you said, maybe Gush won’t specifically make Grow unfair, but we know that GushBond can dramatically increase a deck's power level; cases in point are Tyrant Oath and Gush Painter.  Right now nobody is crying out for restrictions like they were of Flash back then or like they are for the banning of Sensei’s Divining Top in Legacy.  4 Gush Tezzeret with an optional Tendrils kill has the potential to create just such a imbalance.

So I have to disagree with you about Gush.  Unrestricted, Gush has shown it can completely tip the scales of balance in Vintage.  Maybe those scales aren’t completely balanced right now, but at least Vintage is fun again.

Gush should be held up like a counter measure for the right time; a possible alternative to restrictions should any one deck become way too dominant.  And I don't think that time is now.