Because it's the most pro-consumer move and if Sony refuses to do so it'll bite them in the ass. People who buy the upgraded console will not accept to get left behind when the "new generation" comes just 3 years later. This will at the very least disencourage people en masse to buy the Pro versions of Sony consoles.
How is forwards compatibility the most pro consumer move'?
Sony have said a million times that the Pro is a unit designed specifically to make your PS4 games run better and that's it. You should not be expecting anything more than that, this is why it is completely optional in regards to playing the same games as the normal PS4
When they want you to upgrade, you should not expect all later games to work on your PS4 perpetually just because you may not want to upgrade like you've always had to. That makes no sense, and is analogous to what devs want to do as well from a purely technological perspective.
Now if we're talking full backwards compatibility, that is the clear way forward to me. If you want to play the games fully taking advantage of PS5, you can do that, but your library you've already invested in should stay solvent.
I don't think Sony or MS are resetting the clock but maybe changing how generations are handled instead. I know both Sony and MS have been saying that these console refreshes are considered the same generation as the PS4 and XBO, but I can see support being dropped for the OG models before dropping support for the Pro and Scorpio.
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Pro isn't strong enough to be considered its own unit, especially not for long enough to drop support for PS4.
The CPU is only 30% stronger, the RAM is only 30% faster. If your making a game for the Pro, the basic assumption is that a PS4 version also has to be possible by default, as the Pro was only built(from a components stand point) to give a graphics and resolution boost over it to begin with. It is not strong enough to have games that could not be made on the previous hardware, or even give a tangible boost of experience beyond it.