You can still play what you bought on PS even if the stores are shut down. SIngle player games are playable off the disc with no sign in needed. I believe there are DRM issues even if you buy a disc version on Xbox.
I also believe that the Series X version is not on the disc when you buy them, you need to connect to download the data from servers.
So....there certainly is a point in owning your game on PlayStation...more of a point than almost any system apart from Switch right now.
You can still play it, if you have it downloaded before the servers shut off. Sony delist their own games as well, without the entire servers being shut down. Didn't they wait like less than two years before trying to shut down the servers on Gravity Rush 2?
Owning something digitally is a hard sell, due to the nature of licenses, even without bringing in Sony, who are notorious for shutting things down and taking things away.
Sure, it's not much of a better situation on Xbox when it comes to actually "owning" something, but at least there are legacy titles available. I can play Splinter Cell Chaos Theory (just one example) on Series X, if I bough it on 360. I don't know if there is a single Playstation game "purchase" that has carried through from PS3 to PS5. Is there?
Even if there is, most legacy titles aren't available on the platform, whether you bought them at one time or not, while they are on Xbox.
Neither of these situations are really ideal for "owning" something anymore, which sort of blows. That's why I wouldn't hold one above the other when it comes to owning a game. Both are glorified license rentals, but one is a really good value.