Ideally it should just be like how PS1 and PS2 emulation were handled on the PS3 -- just ISOs you download into an emulator that's already in the system. The issue with PS2 emulation on PS3 is that the PS3 wasn't powerful enough to emulate the whole PS2 library well enough for Sony to allow physical discs. People hacked the PS3 to allow physical PS2 games but I think performance was pretty spotty in a lot of games. I remember publishers announcing certain PS2 games wouldn't come to Classics on PS3 because the emulation just didn't work. It's generally assumed the PS4 has enough grunt to solve this problem, so ideally Sony would just allow PS1 and PS2 discs on PS4 while also selling hundreds of classics from those consoles on PSN, keeping the licenses people bought on PS3. I think Sony would have to re-license those games for them to be playable on PS4 though which might be the biggest problem.
This is why ultimately, emulation for physical discs is the best way to get the whole PS1 or PS2 library on there. There are just too many games to try to get on PSN.
I've thought about the OP's idea before though. A one-time purchase of the emulator might make sense if and only if it plays PS1 and PS2 discs. It would be a smart way for Sony to at least get some ROI out of that physical backwards compatibility. I'd probably pay $50 for that. I wonder if you could do it for other consoles though. Like, imagine if SEGA developed a Dreamcast or even Saturn emulator for PlayStation and Xbox that played the old discs, and just sold it for a one-time purchase.
Except Xbox One does it for free and would be a PR nightmare if they charged for it.
The disadvantage of Xbox One BC is it can only play games that are downloadable from XBL. It's basically a digital-only emulator, Microsoft just added the ability for it to read old Xbox discs and know what game to download, using the disc purely as DRM. I don't even know if Sony could do that for PS1 and PS2 games.
Sony selling a disc-compatible emulator gets it some revenue on its own while also letting players potentially have access to the hundreds and hundreds of legacy games that would probably never be re-licensed for sale on PSN.