I dont really see a risk for the long term. I used to think that this game would eventually have a decline in its popularity and was always cautious to sell off my power for the greatest return in the event that it would take a turn for the worst. However, after seeing the majority of what the average person buys in terms of singles, I would have to say that there is no way that this is going to end any time soon. A little clarity, I help maintain the top rated seller on cardshark, and for the most part the stuff people buy are things that are nonexistant on the competitive radar. I get rather giddy when some one actualy buys a legacy deck, or at least the bulk of it anyway. The truth is that there is a MASSIVE crowd of people that play magic outside of the stores we are used to seeing, and infact these are the people IMO that keep the game going.
The only REAL threats/risks of buying power, at this moment in time, happen to be as following: THEFT, FAKES and THE LOCAL INTEREST in the format. If for some reason any of this isnt he answer you were looking for, try asking in a less vague way so I would knwo what you want to know.

Anyway, hope this helped