Don't know about Xbox, but the higher your install base, the less important harware sales become compared to software sales, where the vast majority of your profits come from.
At some point they'll have to, in order to catch up with technology and the demands and expectations of developers and players, but if they have the ability to draw this out and just enjoy the profits, then I'm sure they'd want to do that.
Definitely makes a lot more sense for Sony than Microsoft. I think Microsoft believed their own PR for far too long ("XB1 sales are outpacing 360 sales launch-aligned."), hence why they've been so slow to prepare for the next generation.