Ovinomancer
No flips for you!
This is a subtle bit of strawmanning. I don't deny this is a perfectly valid description of play. However, if monsters are assigned proficiency in social skills, then this is an indication that this bonus is to be useful. If that bonus is to be useful, the the basic CHA bonus is also to be useful. The only way these can be useful is if it's not always certain for use against PCs, because that's the only way to get to those numbers being useful. So I actually rely on the normal loop of play for the argument, I don't discount it as you seem to suggest here.It seems like you’re treating “attempt to persuade someone” and “make a CHA ability check” as essentially interchangeable, and proficiency in the persuasion skill as strong evidence that a creature is intended to be able to do that thing. This is not how I understand ability checks to work. In my understanding, all creatures can attempt to persuade someone, and if the creature’s attempt to do so has an uncertain outcome, the DM may call for a Charisma check to determine what happens. When the DM calls for an ability check (Charisma or otherwise) to resolve an attempt to influence someone with tact, social graces, or good nature, the creature can add its proficiency bonus to the check if it is proficient in Persuasion.
The counter to this is that such bonuses are really only useful against other NPCs or monsters, but this implies strongly that GM solo play is expected by the rules, despite it not being mentioned anywhere. And I say GM solo play because any such usage has zero input from the PCs -- it's only the GM playing between they're NPCs.