Whatever we do with drain there will be another best counter. Then we'll restrict that.
This is simply not true.
First of all, it's a slippery slope fallacy. The slope is not slippery.
There is absolutely no reason to think that if Mana Drain is restricted that another card of the same mechanic -- a counterspell - would be restricted. Legacy has shown that there is no other counterspell that even remotely approaches the power of Mana Drain. The most played counterspell after Force of Will is Counterbalance, and that card sucks in Vintage for obvious reasons.
Take a look at January's data on morphling.de for most played cards:
1. Force of Will (220/55/4.0)
2. Polluted Delta (167/57/2.9)
3. Flooded Strand (135/53/2.5)
4. Mana Drain (129/34/3.8)
5. Underground Sea (121/47/2.6)
6. Thirst for Knowledge (120/32/3.8)
7. Wasteland (107/31/3.5)
8. Tormod's Crypt (101/47/2.1)
9. Duress (97/39/2.5)
10. Chalice of the Void (84/23/3.7)
11. Black Lotus (75/75/1.0)
12. Mox Sapphire (75/75/1.0)
13. Ancestral Recall (70/70/1.0)
14. Mox Emerald (68/68/1.0)
15. Mox Jet (67/67/1.0)
16. Volcanic Island (66/27/2.4)
17. Mox Pearl (65/65/1.0)
18. Leyline of the Void (64/16/4.0)
19. Mox Ruby (64/64/1.0)
20. Pithing Needle (64/32/2.0)
21. Sol Ring (59/59/1.0)
22. Tarmogoyf (59/15/3.9)
23. Thoughtseize (58/22/2.6)
24. Time Walk (58/58/1.0)
25. Tolarian Academy (57/57/1.0)
Look at December stats:
1. Force of Will (360/90/4.0)
2. Polluted Delta (283/96/2.9)
3. Mana Drain (248/63/3.9)
4. Underground Sea (227/82/2.8)
5. Duress (203/67/3.0)
6. Thirst for Knowledge (199/53/3.8)
7. Flooded Strand (195/81/2.4)
8. Wasteland (179/52/3.4)
9. Tormod's Crypt (161/85/1.9)
10. Chalice of the Void (127/35/3.6)
11. Black Lotus (125/125/1.0)
12. Pithing Needle (118/56/2.1)
13. Mox Sapphire (113/113/1.0)
14. Mox Jet (112/112/1.0)
15. Leyline of the Void
15. Leyline of the Void (111/30/3.7)
16. Volcanic Island (110/44/2.5)
17. Dark Confidant (109/28/3.9)
18. Mox Emerald (106/106/1.0)
19. Mox Ruby (106/106/1.0)
20. Mox Pearl (101/101/1.0)
21. Ancestral Recall (100/100/1.0)
22. Red Elemental Blast (94/45/2.1)
23. Sol Ring (91/91/1.0)
24. Tarmogoyf (91/25/3.6)
25. Thoughtseize (91/32/2.8)
Mana Drain is the SECOND most played
Spell [/b] in Vintage Top 8s. Not counterspell. It's the second most played SPELl.
Even beyond that, Mana Drain is the third or fourth most played CARD in Vintage top 8s. That means that Mana Drain has more top 8 appearances than basic Island or Underground Sea. No one can possibly think that's healthy.
Take a look at May, 2008, the last full month before the June 20 restrictions took effect:
1. Force of Will (384/96/4.0)
2. Brainstorm (380/95/4.0)
3. Leyline of the Void (354/89/4.0)
4. Polluted Delta (294/95/3.1)
5. Underground Sea (243/90/2.7)
6. Merchant Scroll (220/72/3.1)
7. Flooded Strand (216/85/2.5)
8. Duress (188/72/2.6)
9. Gush (186/48/3.9)
10. Tormod's Crypt (148/72/2.1)
11. Wasteland (144/43/3.3)
12. Ponder (143/47/3.0)
13. Black Lotus (133/133/1.0)
14. Volcanic Island (129/59/2.2)
15. Red Elemental Blast (128/55/2.3)
16. Mox Sapphire (127/127/1.0)
17. Tropical Island (125/73/1.7)
18. Mox Jet (123/123/1.0)
19. Chalice of the Void (113/31/3.6)
20. Thoughtseize (113/52/2.2)
21. Mox Ruby (109/109/1.0)
22. Mishra's Workshop (108/27/4.0)
23. Dark Confidant (105/28/3.8)
24. Ancestral Recall (102/102/1.0)
25. Extirpate (102/54/1.9)
In terms of appearances in top 8s, Mana Drain is CRUSHING Merchant Scroll, Gush, and Ponder. The only card it is comparable to is Brainstorm. And ALL four of those cards were restricted. If those cards were restricted for producing lesser dominant decks, lesser dominant engines, AND saw much less play, it's hard to make a case that Mana Drain isn't worse than all of those cards.
As to your point that another counterspell would be restricted if Drain were, well, the third most played counterspell is Red Elemental Blast. After that, waaaaay down the line, is Stifle. It seems very unlikely that if Mana Drain were restricted that Red Elemental Blast or Stifle would ever see restriction. Neither card is a superswingy tempo card in the way that Mana Drain is, giving you a burst of colorless mana on your next main phase.
Then something else will spring up....and we'll restrict that. for whatever reason we always come back to the opinion that at some point we'll restrict enough stuff that we'll end up with white and green decks that are viable, and it's just not going to work that way. Before we get to that point we'll pass the threshold of a restricted list at critical mass and be playing highlander.
As I said, the slope is not slippery. I do agree that there is a danger that a certain number of restrictions could create a cascading restricted list in the sense that more restrictions make it more likely that other restrictions will follow, but there are three responses to that.
First of all, that does not mean that they will be. History shows that one restriction does not lead to the result of further restrictions. For example, the restriction of Trinisphere did not lead to further restrictions. The restriction of Lion's Eye Diamond and Burning Wish did not lead to further restrictions. The restriction of Fact or Fiction did not lead to further restrictions. While more restrictions did eventually occur, that was not a result of the previous restrictions. It is likely, simply as a matter of historical statistics that another card, someday, will be restricted. But that isn't an argument against restriction.
Imagine if the DCI used that standard for Extended. They would say: we can't restrict this card we need to restrict because we may restrict cards some day down the road. It makes no sense.
Second, it's really too late to put the genie back in the bottle. The DCI has restricted Gifts Ungiven, Brainstorm, Ponder, Flash, Trinisphere, Merchant Scroll, and Gush for creating decks that were weaker, less dominant, and less played in Top 8s than Mana Drain.
Third, that's why I think that the best solution is not to restrict Mana Drain, if Mana Drain's dominance persists, but to unrestrict Gush, Ponder, and Flash, to help build decks that can compete with Drain decks. Alternatively, restrict Thirst first. All of the other major blue draw engines have been restricted, and Thirst for Knowledge is better than Fact or Fiction. Why not Thirst?