What you’re saying doesn’t make any sense - If the professors are teaching 95% of the classes and grad students only do grading or basic work, how do grad students get the teaching experience they need to be competitive in the academic market?
Why would Penn ask their grad students to teach less and do less hands on classroom work then what they would find at other top schools? Moreover, how can Penn convince their faculty to teach more than they would at other top schools, and give the grad students less to do?
This would put Penn at a competitive disadvantage both for faculty recruitment AND graduate student recruitment. That doesn’t seem to make any sense at all.