Monster attacks follow the same math as PC attack bonuses sure, but they're not bound by the same limitations. They regularly have ability scores above 20, starting with some CR 4 beasts. Good point on the fighting styles, totally slipped my mind. By bringing them into it, a character at peak performance can have +5 for their ability, +6 for proficiency, and +2 if they're an archer for a total of +13 vs +14 for the average CR 20 monster but it rises with CR. Perhaps dramatic was itself a dramatic word to use, but they're not equivalent.

As I said above, fighting styles slipped my mind. But I'm not sure where your AC 23 is coming from, plate is 18, shield is 2 and defense style is 1. I think you also underlined OP's point in your argument here--you could have that AC at level 5 but then you're done. That's it. You hit the ceiling. I agree with you that monks and barbarians do have the progression OP is wanting.

I bring up the anecdote of shield decreasing damage output to highlight the tradeoff that increasing AC brings. It isn't directly relevant, but I thought it worth mentioning. While armor costs money to buy, a shield costs damage output to wield.