An RPCS3 contributor explains that native PS3 emulation is possible on PS5, but isn't worth the effort for Sony.
PlayStation Plus Premium will give players access to a library of classic PlayStation, PS2, and PS3 titles starting this June, which is exciting as the PS5 only is backward compatible with the PS4 currently. Unfortunately, it comes with a caveat: All PS3 games must be streamed from the cloud and can’t be played natively on the console.
CELL it The system architectures of the PS4 and PS5 are so similar that isn’t much of a problem to run PS4 titles on Sony’s latest system, with a couple of exceptions. Compared to that, the PS3 has a different CPU that has more in common with the PS2 than Sony’s modern systems. The uniquely powerful CPU of the PS3 already makes it an odd system, but the PS3 could also move 128 bytes atomically and had a weaker GPU than the Xbox 360. This combination led to a weird post-processing workaround for developers that Whatcookie called “unfriendly to emulation,” where developers would offload post-processing to the CPU.
“The PS5’s CPU is a decent deal faster, and combined with the kind of shortcuts that developers of commercial emulators make — the official PS2 emulator on PS4 has many game-specific patches and hacks — it should be possible to achieve full speed on whatever games they choose to release.” Whatcookie points out that Sony only needs to emulate certain games so that investment might not be worth it.
In an age where high-priced remasters and rereleases are coming, creating native emulation for these games on a service only some hardcore fans will use likely doesn’t have a great return on investment. As Sony already has pre-existing PS3 cloud gaming tech with PlayStation Now, Whatcookie points out that going back to the drawing board with emulation isn’t worth the effort.