Well, you say lateral movement is fundamental in mma - which i agree with, but its precisely this extreme obsesion of Wing Chun with the center line that impedes lateral - and general - movement.
When you start to move and make powerful changes of direction your hands must change accordingly, at these high speeds your body contains a lot of different inertial vectors. When entering in range you take what your body is giving you at that precise instant and use - in part -those vectors to throw your hands (hooks, uppercats, crossed,etc). WC forces you to not only to attack the center line (which is indeed the most basic principle of chinese boxing) but to do so along -and only along - the line between centers - i.e. wingchun straight punch.
This is, in my modest, ignorant opinion, the main reason WingChun wont work in MMA sport (where theres a lot of time & space to move). Trying to attack along that line only would disrupt your body aligment when making diagonal changes at high speeds. (Try to make powerful diagonal steps while mainting the WC guard and yoou