The only reason I can think of to keep my PS4 Pro around is that it's the only way to see the latest DLC/addons every week in a single list. As far as I can see, PS4 PSN is currently the ONLY way you can still do that without going game-by-game.
Honestly, not having the ability to see all the latest DLC/addons on the PC, PS App or PS5 PSN in a single list just seems friggin' ignorant on Sony's part. I mean, they do WANT to sell that stuff, right? They get a percentage of those purchases, right? So why not make them easy to discover? There's quite a lot of little "hurr-durr" things like that about PSN.
Other than that, I don't really see any other reason to keep my PS4 hooked up anymore.