It's not a new game, it's an update. A big one, I agree on that, but "just" an update none the less.
I know some things like the improved AI must have taken a lot of time and tweaking to achieve what we're going to see in this release.
But it doesn't seem like a "3 years" kind of workload. I don't know how many devs are behind LSPDFR but I imagine just a few as it took 3 years.
If they don't have enough time to work on it, nothing stops them from getting more people involved. The community for this mod is huge and I'm sure a lot of very competent developers dream of working with them to improve the mod.
With that said, I have a huge respect for those actually working on LSPDFR. A big community means a lot of pressure and expectation, and I'm sure they do their best.