I think the interesting point here might be one could be able to extract some secret from memory of a PS5, like to break some kind of encryption

Interresting, could well be a path to jailbreaking the PS5... although, not sure if that has or hasn't already happened. For XBox Series, you can just use dev mode in the first place.

What valuable secrets do people have on their PS5/Xbox? You also need a way to deploy the malicious payload on those platforms which, due to their closed nature, is very difficult to do.

The valuable secret here would be the keys that let you decrypt and copy games. The threat models of locked-down platforms are incredibly strange.

That's a good point but I can't believe that every console doesn't have it's own unique set of keys so that if you compromise one before SW patches land, it won't be much use in the ecosystem.

It depends. I'm going to speak in general terms, since I obviously don't know how every single system works, but per-console keys are used for pairing system storage to the motherboard and maybe keeping save data from being copied from user to user. Most CDNs don't really provide the option for on-the-fly per user encryption, so instead you serve up games encrypted with title keys and then issue each console a title key that's encrypted with a per-console key. Disc games need to be encrypted with keys that every system already has, otherwise you can't actually use the disc to play the game.

As for the value of being able to do 'hero attacks' on game consoles, let me point out that once you have a cleartext dump of a game, you've already done most of the work. The Xbox 360 was actually very well secured, to the point where it was easier to hack a disc drive to inject fake authentication data into a normal DVD-R than to actually hack a 360's CPU to run copied games. That's why we didn't have widely-accessible homebrew on that platform for the longest time. Furthermore, you can make emulators that just don't care about authenticating media (because why would they) and run cleartext games on those.

At least with the PS3, I seem to recall that I couldn't extract any of my games' save data from the hard-drive of my PS3 unit that went dead due to RROD (or was it YLOD?) because the hard-drive was encrypted using the PS3's serial key as part of the encryption.

I don't know if that mechanism persists into the PS4/PS5.

Oh, I can imagine lots of uses for a bevy of PS5's, assuming you can gain remote control. What do you do with a botnet? What do you do with a botnet with a pretty good GPU? What do you do with an always-on microphone in people's living rooms?