The power duration curve for most low level roadies, like me, is such that it goes down quick from 20min to an hour. 20min is closer to the intervals you often do. 5x5min, 3x8min, 3x12 under over.....etc......
I do agree, for you to solo a 4/5 race you need to first get away and hold VO2 for a few minutes then settle into your effort. Some can do that, some can't. That's actually exactly how the shot off the line works in Zwift. VO2, settle into 25min of threshold, pray for a miracle sprint.
The original issue though is Zwift Cat C is up to 3.1w/kg for 20. B is up to 3.9w/kg for 20. If that's the case, having folks in C doing 300w for 30ish min just seems ludicrous for the limit to be 3.1. I can do 4.0w/kg for an hour today. I'm like 70kg. So, only 280w.
So, goes to my idea that it's likely B and A riders sandbagging down by adding weight.
Totally agree with the above that once you get to certain fitness, if you ain't 7 feet tall the weight will come down at least a bit.
Personally, I haven't raced Zwift due to this. I'd class into A immediately on w/kg. Even on uphill stuff, since A goes from 4 to 4.5w/kg I'd get roasted.
Basically..........go to a points system instead of w/kg or watts. New guy wins? New guy moves up.