Today, there is 20+ games available that seem to work almost perfectly with no performance impact from running in emulation (some appear to be better in fact... ie ME loading times).
We also know that several big AAA games this fall will include the 360 versions of the previous games along with the Xbox One version, presumably using this same BC emulation since it is basically "free" for them to include them.
MS has said they intend for there to be 100+ games available for public launch this fall, and if it really is as easy as it seems to add the games given the emulation is already working, then I see no reason why that wouldn't be true.
Will 100% of their libraries be compatible? No, of course not. But a large portion very well could be.