[2023] Armour Vs MR - Riot got their Maths Wrong?

I trust you've done a lot here, however there are a lot more factors, as you're alluding to.

A significant portion of all physical damage taken is from auto-attacks, which are on average more multiplicative, since AS and Crit multiply them. Plus turrets and minions immutably deal physical damage. The amount of physical damage taken actually exceeds the amount of magic damage taken in most cases, which makes Armor an inherently more valuable stat than MR, even if you're getting less % gains per gold. Good luck actually doing the analysis, but the crucial missing factor is relating the math from this post back to damage actually taken.

Regardless, component costs aren't terribly important. Serrated Dirk is an intentionally over-efficient item. But you can't stack Serrated Dirks. Youmuu's fights Luden's, Prowler's Claw fights Shadowflame, etc. Similarly, Randuin's should be compared to Force of Nature. No one stacks cloth armors.

In general, I'm sure we have a ton of outliers regarding base stats and ability scaling at this point. It's not something I think we've tried very hard to constrain. At the end of the day, if we want to buff Kassadin and don't want to increase his damage, there aren't a lot of ways to increase durability if the only stat we're allowed to touch is health. Not saying we've treaded perfectly here, though.