If they opt not to make it backward compatible, there would be no good reason other than laziness or greed. I understand why PS4 wasn't backward compatible with 3, because the PS3 ran Cell architecture whereas PS4 was x86, so likely the only way they'd be able to make it work would have been by plopping down a Cell CPU in the PS4. Both PS4 and PS5 are x86 based, so there are no architectural limitations.
Agree, and Sony's been there before (removing PS2 BC from PS3 'because there are enough games on PS3 now'). They've got PS Now they're pushing a bit harder, which could influence development decisions as they watch competition come online this fall that'll probably be more successful out of the gate than their service ever has been.
You'd hope this would be one of the things that would be a little more definite at this point, granted we're a year out.
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