That's only if you leave the Sherman on HE rounds. Which I always do and anyone in their right mind does. (Actually I use the Scott, Which is also kind of hinting at the problem too.)
The timing of the Sherman just doesn't really fit into being much of a shock unit either. It's around the time of Panzer 4s and has the choice between being able to kill infantry or sorta match with Panzer 4s.
I get what you mean though, that's definitely the supposed role of the Sherman. I just don't think it really plays out that way. The Jackson and Sherman are usually competing for resources more often than not and the one with the more critical role (AT) wins out.
USF's whole advantage is beating down axis infantry.
Scotts are underrated and are better for "All In" assaults than Shermans, Shermans just scale better, and make wonderful tank "blobs".
I feel it fills the niche very nicely if you can get a fuel lead, plus it can discourage sending units to ninja cap, it's one of the best USF units for that job, forcing the enemy to spread their forces and giving you ample angles of attack.
I could see people getting a Jackson first, then massing Shermans for USF lategame, which I'm really excited to try.