Space says:
after 4,5 years being a trackmaniac, I'd rather have a new SM enviro I moved to the shooting side...
And yet, the other thing that occurred to me is that even if they incorporate mixed environment servers with trackmania, it won't work nearly as well with shootmania. Following the pattern that each environment has it's own specific car, the new shootmania environment would have a new player and new physics, so unless they keep the same physics, a new shootmania environment will basically be a new game.
Though I never played multiplayer in TMUF, so I can't really speak firsthand...