PoP is a fantastic mod, but you can't do anything of that scope until you have a finished base game. If you go to their subforum you will see that the developers of that mod themselves wouldn't really want to work on a port to Bannerlord while the game is still this fluid.
I agree that PoP is amazing and the team is fantastic. But people have to remember that it is WAY easier to create something if most of the hard work is already done, like... making a whole game. They didn't have to build an entire engine from scratch, they didn't have to create most of the art, or the programming, UI, AI, etc, etc.
So to say that PoP is better because it's more "polished" than Warband is insulting to the team that created the game that allowed that mod to exist in the first place, and all the hard work that went into it.