Concerning the bulbous front element:
At least Zeiss is able to build an excellent 15/2,8 lens without a bulbous front element. Well, but it's a prime lens and not a zoom lense.
ok, yes a couple of things:
1) prime as you stated so totally different
2) not bulbous but requires 95mm filter - 16-35 II 82mm
3) no autofocus
I know for an event photographer likely all three of these compromises would be a deal killer. Canon's last improvement to the 16-35 included increasing the front element so that it required 82mm filters instead of 77mm. While further improvements could be made likely by going beyond 82mm, question is do people want this for event photography/reportage as some already complain the current 82mm is too big.
might make more sense to focus on landscape who would probably prefer wider than 16mm and wouldn't care about front element size/shape.