We always complain about BC, but I just haven't found it to be an issue. As much as I think now that I'll want to play my X360 games in five years, I really won't.
There have only been a handful of Xbox games I've wanted to play since I got my 360, and almost all of them are available in the Marketplace. Ditto PS3 (except that in this case, they're also available as HD remasters). I still have my PS2, and there are a couple PS2 games I've played since getting my PS3, but my PS2 has stayed unused on a shelf for like 97% of that time.
So next generation, I'll do the same thing. I'll keep my 360 on a shelf somewhere, and on the very off chance that I want to play a 360 game, I'll haul it down, plug it in, and play it. Then put it back on the shelf.
In any case, I think we'll see the same thing next gen that we saw this one: games available on the marketplace, or re-released compilations of older games for discounted prices. Either way, I don't anticipate a huge problem.