I think collecting plays only a minor role in preservation, mostly as a steward of these physical trinkets that many years down the road, will be seen as novelties of the past. As with anything which stops being mass produced, it's true eventually the number of surviving copies will diminish over time. Seeing as game collecting is so huge, I think a lot of this stuff is going to last a very long time.
That aside, collecting has little to do with the actual preservation of the media itself. That is something entirely different, and already well archived and preserved through emulation, ROMs, etc so there's slim chance it'll ever be lost.
I think media preservation is a worthwhile endeavor, but only in pursuit of that which is actually in danger of being lost. 99% of video games are not that category, outside of unfinished projects, betas, tech demos, etc. A lot of that stuff is just not in the hands of people who thought it was worth saving, but it does surface from time to time on an old development kit, test cart, or hard drive. Usually, people will dump whatever data it contains onto the Internet and there you have it. That's preservation. As a rule, anything you can readily buy at a store, is already well preserved online.
