The difference between XO's BC allowing discs and PS4's BC most-likely not supporting discs certainly come down to how those discs were formatted. On 360 as well as PS3, the console would recognise what 360 or PS3 game was placed into it before it was launched. On PS3, placing a PS1 or PS2 disc in there would only come up as "PlayStation 1/2 Format Disc", which indicates there's nothing on the disc that the OS recognises as an identifying characteristic. So while the XO can use 360 discs as copy protection for a download version, there's no indication that PS4 would be able to do the same for PS2 discs.
I know lots of people don't care about trophies, but incorporating them into PS2 Classics would still be an endeavour, so regardless, it still makes sense to charge for the digital downloads of them, especially since the evidence points out that there is no general-use emulator, but a system where different versions are used to play to each game's specific case. All of this requires a lot of testing and QA, but judging by the Digital Foundry article, it seems to produce excellent results.