http://m.ign.com/articles/2017/10/23/the-untold-story-of-xbox-one-backwards-compatibilityCredit where credit is due, they are doing phenomenal work.
But I think you're putting too much weight in their forward planning.
If it was always their intention to have these games running on the One they would have at least mentioned it at launch if not had it implemented.
We'd be looking at different console sales right now if they had. While bc is not crazy important for sales overall it makes a big difference early in the gen.
This is a good read in general, but specifically they talk about the BC plans at launch