My reason for this thread is that I was seeing if it was justifiable to pirate PC versions of PS4/PS5 games I already own, in case I were to get a Steam Deck. Is it 'legal'?
if it's the same game and platform you already bought it from... maybe it could be justifiable, same game, same account, same platform. (even if it's not justifiable if you have 2 console and the game is tied to the console instead of a user account, you are supposed to buy it again)
If it's another platform, and the game is not crossbuy and you are supposed to buy it again on that plateform to play it, then no, you are supposed to buy it again.
The devs worked on making that platform compatible, they paid for the SDK and to be allowed to release their game on it. it's not the same "binary".
I would find it "justifiable" (not legal), in case you buy the game but you can't install it. for example, your console is banned !
you still buy it officially to send money to the devs, and get the game elsewhere (or on official servers) to install it yourself. it's the exact same game's version/platform it was intended for, and you bought it specifically for that combo.
OR
the official server closed (like Wii shop for 2 months) without notice, you don't know if it'll open back (it did) and you can't download and install your bought game using official methods.