I only use RetroArch on PC, so I don’t know how it works on iOS, but I will assume will be identical to the console versions, which already have all the cores included on the emulator (on PC they are downloaded individually).
I wouldn’t call it just a front end, it’s much more than that, and it has a great integration. Also the devs seem to focus a lot on the emulation accuracy.
Even though you dislike online play, you can still play online with the PS3. Some servers are still up and running, and for other titles there are custom servers created by some fans. And the best part: you don't need to pay the online fee that Microsoft created during the 360 days, which Sony and Nintendo followed after.