Why would UFC invade?
Even if they dog stop the WWE guys, they still are associating their "legit" (just like boxing, you can't tell me they don't work outcomes) sport with "fake" wrestling. That would absolutely hurt their image.
If they lose it's even worse, and WWE going over is the only way Vince would go for it.
Just literally makes zero sense
All that makes perfect sense...BUT. You have 1 1/2 crossover stars as of right now with Brock and the tease of Punk...they may see it as a money grab for the UFC fighters to try their hand at "entertainment".