Here's the thing: first, PS3 emulation is completely off the table. It can't be done, although there are dozens of crossplay games out there already offering some form of taking your library forward when upgrading from PS3 to PS4. So the real comparison here is between MS providing Xbox OG emulation and offering PS1 and PS2 emulation.
There's an enormous factor that often goes overlooked here: The PlayStation back library is exponentially more valuable than the original Xbox library. The only Xbox games that could move the needle in the market are the first two Halos which have already been remade and resold for the Xbox One. For both the PS1 and PS2 there are many, many games with huge sales potential as HD remakes. People aren't lining up to pay $50 to play Blood 2 or Black or Fusion Frenzy again. But they are for games like God of War, Metal Gear Solid, and every 3D Final Fantasy.
Do you think Square Enix would be happy about full disc emulation with high resolution rendering of their entire PS1/PS2 catalog on the PS4? They'd be fucking pissed when they can sell a million copies of FFX and FFXII in HD. They're even making bank on $20 ports of FFVII and FFIX, or PS2 on PS4 versions of Star Ocean games. Letting Sony give that shit away blows a hole in their nostalgia market.
So while I'd love it if all the PSOne and PS2 classics I've bought digitally and any PS1/PS2 disc I might have could work on a PS4, I think it's an open question as to whether Sony could get that past their publishing partners these days. It's a lot of time and money just trying to negotiate all the required licensing deals, let alone trying to test and support emulation for ~10,000 games. All for a checkbox feature that would have an imperceptible impact on their market share.