The more I think about it the more it seems like a really terrible idea. The only camp that seems pleased with all this is those with disposable income that welcomes - what will be seen as - shorter generation cycles.
Yeah, the games need to work on the PS4 and so in most cases probably won't affect those not willing or not able to upgrade, but if the new console does offer some tangible benefits then the old PS4 will indeed begin to be seen as 'last-gen'.
It's not just about hardware performance though. Back when GTA IV was released you probably wouldn't believe that GTA V was possible on the same consoles.
If we are to see new hardware being released more frequently and support for older iterations of it dropped then we'll inevitably see less of each platform's true potential.