I'd bet with a very few exceptions, old games aren't going to be patched to work with a Natal control scheme. Too much cost, no real profit. Besides, why patch old games when there are new games people will actually pay for?
Software will decide whether this is a success. As has been mentioned before, Ubisoft has 10 titles in the works for Natal, so this isn't an MS-only push.
I'd also be curious what kind of support for Natal will occur in the XBLA space...
Natal games aren't *necessarily* going to be core gamer experiences, so the lack of a CPU on the Natal device shouldn't cause too much hardship. I'd bet that most non-Gears/Halo/COD games do not use all of the CPU cores - Star Wars Lego/Kung-fu Panda type games are the level I'd target for Natal games, which are intended to appeal to casuals anyway. I'd bet those types of games wouldn't have a problem even if an entire single core was devoted to Natal processing.
We'll see closer to E3 2010 what MS has up their sleeve.