You are right. It is subjective. I understand your points, but I think that if you already have a 360, for example, then waiting for XONE to get BC before buying one is odd. You will still have your 360 to play all your last gen games on. The justification in the cost of a new console, for me, is the upgraded hardware and games library.
I had a 360, got rid of it, but still own plenty of games digitally. Games that should I be able to do so conveniently, I would like to revisit at some point in the future.
You should stop making declarative statements without considering a broad range of circumstances and perspectives different people might have.
There's also people who never had a 360 in the first place, people who's consoles broke out of warranty (a sizable portion!), people who got rid of their 360 for financial/practical reasons (this is me, I got rid of it because I was moving halfway around the world).
It's as viable factor in people's purchasing decisions as any other factor. I don't know why you have such a reductive perspective on it.
I've moved on, and it wouldn't be the end of the world if I never owned a device that could play 360 games ever again, but hearing that BC existed for the Xbox One was probably the first time I have ever thought "I might buy one of them then". There are a couple of exclusives I am interested in, but not enough to justify buying a console for in isolation.