About LLE emulation

So is that it really? Execute some cellSpursFile.elf or whatever and you will have The Last of Us running with some improvements to RSX? Can't be that "simple" surely? (What I mean is that looking at the current state of rpcs3, is that really the only big thing left now?)

It is "that simple", but it requires that the hardware is emulated accurately enough to run those modules without errors. That's the hard part. (This includes RSX, like tambre says, which is the hardest part.)

It's also really hard to get that right because parts of the hardware are largely undocumented (mainly RSX) or contain small, hard to track down errors (accuracy errors in the PPUs or SPUs for instance.)
Emulation is kind of hard.