There doesn't seem to be any evidence supporting BC as a system seller outside of the launch of a console because of the lack of games during that time (and even then, that's arguable). It's always been something that only matters to gaming enthusiast.
I mean, Microsoft is only doing BC right now purely as a PR move in their continued effort to recover from their XBO reveal. Nintendo has made reselling you games a core part of their business model and Sony abandoned BC back in the early years of the PS3 and by all accounts, that didn't hurt them in the slightest. Also, the sales success of re-releases, remasters, and remakes prove that companies would just be leaving money on the table by giving that stuff away for free.
Maybe with console hardware using more traditional PC parts, BC can be done easier but I'm not a programmer or engineer, so I have no confidence in saying that. It's often stated though that making games BC takes a lot more work than many people on gaming forums give credit to and to do that work to then give away the game for free just doesn't make a lot of business sense.
Also, I'm not sure why people keep bringing up games being digital. The only difference between a digital game and a physical one is the medium in which the content is delivered, the code is the same. I don't see how digital games are somehow more capable to be backwards compatible.