My take:
Microsoft was never going to win the generation. They are never going to outsell PlayStation. Their best shot was the Xbox 360 and even then, with a year's headstart, they were only able to stave off Sony until the 8th gen began.
Launching at the same time, with weaker hardware, and a higher price? No contest. With that said, M$ had done a hell of a job giving its users features. Console selling features? Ehhh....
As most people know, we buy new hardware for new experiences. At the same time, why would anybody not want access to old games? It's like buying a Blu-ray player and saying "I don't want it to play DVDs".
Even if I just fire up an old game once or twice a month, that's still several hours of enjoyment that I'm still getting. I still have a fully functioning Xbox 360 right next to my Xbox One. I can play any game I want at any time but I take full advantage of backwards compatibility on the XBO--mostly because of the controller and other bells and whistles. I also still have a PS3 right next to my PS4. I still buy older games on that platform--usually digitally.
Given the option, I'd love BC on PS4. Thinking back to E3 2015, I'd say I've personally used BC on XBO more than any of the games that I saw announced from Sony that year--and some of the big ones STILL haven't come out. For my time and my money, backwards compatibility won E3 that year. It wasn't as exciting as a new Shenmue or Final Fantasy VII but in practice, it's something I've gotten a lot of use from.
