It's extremely more likely that they'd put up emulated Nintendo games. We have something bordering on perfect emulation for several of Nintendo's old systems, and we don't have that for Xbox. They can literally just slot in an emulator that someone else coded rather than having to patch in custom emulation code on a per-game basis like they currently do for backwards compatible Xbox games. Again, the point is moot. Microsoft will not own Nintendo, but if anyone else took over Nintendo for any reason, it's much easier to sell fully functioning retro Nintendo games on PC than it is to do the same on PC for old Xbox games.