(and it can still beat PzIVs cost efficiently.)
The ROF was an obvious issue so they started from there (when they should had probably redesign the two units separately).
I think these are fair enough points but I still think OP's original point stands. (At this point it's pretty pie in the sky to think a unit rework will happen). "Medium Tank that's slightly better than stock M4A3 at beating P4s" just isn't a strong selling point.
Pre-nerf Mechanized was mostly used for Dodge Truck pressure and Dozer Shermans as far as I can remember. Meanwhile the ROF was the only real thing that made the 76MM somewhat attractive since it made up for the lack of HE shells when you weren't using HVAP (do people even use the HVAP on 76MM?). 76MM needs some semblance of AI perks otherwise you might as well get a Jackson if you want to tame P4s. This is even more true now that Mechanized's power level was nerfed pretty hard is other areas.
TL/DR I don't think a slight buff to 76MM's standard shell will put it over the top given how the rest of Mechanized was nerfed if it's going to be one of the main reasons you pick the doctrine.