The 21 AC/defensive was literally to make math easier.
A fixed AC is correct, because we are measuiring character power. Measuring it against similar level foes convolves character power and monster power. So I picked the simpler option.
21 ac and crit immune means avarage damage on a hit times accuracy divided by 20 is your damage per swing. And that is the same ac as the simple character.
If you lower AC, this causes a larger percent increase to low level fighters compared to high level fighters. Which makes it more convex down.
Keeping things simple is about jnderstanding the math. I can do this in a spreadsheet or a half dozen programming languages, but the simple model is both sufficient and easier to understand. I can also understand what happens when you tweak it, like lowering AC, because I did the math.
DPH*HITPIPS*APA*APD/20 is your damage per day. Hitpips go from 5 to 11 against 21 ac; adding a constant corresponds to lower AC, and bends the dpr curve down. Having AC scale upward with level also bends the curce down.
Static damage and accuracy from magic weapons booses DPH and PIPs. As DPH starts at 10 and goes to 12 baseline, each +1 damage is a 10% upward bend. If we scale AC down by 5-7 and have it go up with level, that more than makes up for thd +3 to hit and damage curve from weapons.
Which leaves feats. The -5/+10 feats let you convert advantage and/or runaway ATK bonus to DPR, and give you more ways to convert ASIs to offensive power.
Adding the rest (feats, magic items) is optional. If you claim that magic items boost fighter dpr more than spellcassters, or fightera have better feat support, go ahead. But those are specific problems with optional rules and things 100% under DM control.
AL may have super linear damage output, but they use lots of optional rules, and strips DMs of much of their toolkit. It is D&D lite.