I think retro game collecting will always be a popular thing to do, but what people will call "retro" will move to newer systems. The NES and SNES will no longer be in the spotlight because no one will have grown up with it anymore.
While there are those little snot-nosed brats on YouTube who are less than half as old as the games they're collecting, most of the retro game market is driven by nostalgic guys and gals in their 20's 30's and 40's who are looking to find something fun they used to do when they were kids. So in 5 years, N64 stuff will be considered retro and get pricey, in 10 years it'll be PS2, Gamecube, Xbox, etc, and in 15 years it'll be the 360, PS3, etc.
This is why I'm focusing on N64 stuff now since the prices are so crazy low - I'm almost done collecting my personal "greatest hits" and I've spent less than $50. PS2 stuff is also at rock bottom right now price-wise - I don't think I've paid over $4 for a single game, save FF10 which was $7. I never had a PS2 until two years ago, but I remember playing the games with friends in highschool and when they're under $5 a pop, it's hard to say no even if you only played it once.