Reducing the effectiveness of stock "heavy" TDs vs mediums can and probably will increase diversity greatly.
Exclusively if you have something for the meds.
USF, for reasons already mentioned by Sander, do not have these alternatives.
There is no "heavy TD" for usf.
There is just THE TD, they have no alternatives without doctrinal commitment.
If in your definition a TD being able to penetrate a tank 100% of the time regardless of range is overpowered, then we also have to nerf StuG, which pens all allied stock meds 100% of the time at all ranges, while having better reload.
Does your argument exclusively focus to allies, or extends to axis as well?
Because OKW for example almost exclusively rushes panther with JP4 being used extremely rarely.
How would you "fix" that to increase diversity?
Tanks are not supposed to be effective and efficient against TDs, its supposed to be the other way around.
I can't see why you struggle with that concept so much.
Bigger TDs are being used, because they are reliable.
They will always be, because they are supposed to.
The last time we've seen anyone go for non top tier stock TD was when KV-1 was a call-in as SU-76 was used to support it, not so much reason for that and you see 85 behind it.
That means you can't increase diversity without making better option inefficient in some area, in which case you will not introduce diversity, you'll just introduce underpowered unit.
And it counts double for a faction that has no alternative at all and just 1 vehicle to deal with all armor.