I've argued it before, it simply makes sense for them to make the PS5 hardware capable of playing PS4 games because of PS Now. That way they can utilize only PS5 hardware in the datacenters instead of requiring PS3 hardware to still be manufactured, installed and maintained as they're currently doing. The incentive for them would be the many, many millions of dollars they'd save by finally killing off Cell production and having to only support a single unified platform.
Now since I last made this argument Sony has of course signed their MoU with Microsoft and are looking to use Azure for PS Now. We have no idea how that partnership will go or how it might affect the points I've made, so I'll stick with this theory until we get actual info that would negate it.