Well one of the alternatives being analyzed is BRT in the trench, and I think the idea is that a successful rapid line off of Lake St would mean that fewer 21s would need to be run. Certainly any space that opened up on Lake would just be filled by more traffic, though, so I agree that reducing congestion isn't a realistic goal. I would hope that the primary goal is improving the transit experience for the thousands of existing daily riders and that a secondary goal is increasing transit ridership in the corridor. Why should every transit project need to benefit motorists? Certainly most roadway projects do nothing for transit riders.It can't replace 21/53 or really reduce frequency.
Unless trench was used as alternate & exclusive busway for those two lines, (which no one is suggesting, I don't think) it won't improve University traffic or line efficiency.
Instead of wasting their time studying a streetcar on Lake St itself - which is doomed to sluggishness - they should study rerouting the 21 to the trench. Doubt the politics will work with that idea though.