Your forgetting a very important factor in reviewing crit builds: Luck.
You cannot assume mathematical averages will actually happen. It's frankly
unreasonable to assume they will.
For a player like me, a person with substantially bad luck who will always get bad results from any chance based game mechanic, the choice isn't "diminishing returns vs use different option", the choice is "maybe crit sometimes or literally never crit".
KotUMK, Bloodthirst, Deadshot, some cape I don't remember the name of, and a helmet that's a spoiler.
From what I remember in equipment, you could stack your critical roll down to 15 (just from gear). So 25% crit chance. Make it 30% using the crit elixir.
So you'd think in theory one would crit 25% of the time. In practice, they only way I'd have any hope of critting at least 30% of the time is if I had 100% crit chance.
Every source of improved critical lowers your roll by 1, which raises your chance by 5%.
If going from 0-5, I should just stay at 0 because I'll still literally never crit. If going from 5-10, I should stay at 0 because I'll still literally never crit.
If I don't have at least 20% chance, I have 0% chance. Anything below 50% is 10%.
You posit that relative increase begets diminishing returns, meaning that that 5% is worth most when you have none and the more you have the less it's worth.
I posit that because of luck, and the premise that what actually matters is your net total chance, not relative comparison; a 5% increase only actually matters if I already have enough (20%), and matters
more the higher the total gets, except for the outlier where the most important chance increase is the one that hits/crosses the 50% chance threshold.
I would never do a crit build without including at least 3 fighter levels for Champion, because that extra 5% is needed for the attempt at a crit build to feel even remotely worth it. A crit build without champion is one where I don't get any crits.
This isn't just thoughts either, I've been getting it in practice.
My most played character so far (and yes, one that's finished the game) is a Goolock Bladelock, because that's just what I chose.
I had decided I wanted to capitalize on Mortal Reminder, so I stacked the items listed above (but replaced one weapon with the weapons that get buffed by being a pact weapon), the Risky Ring for constant advantage, 6 levels in fighter to take Champion and use the fact that Extra Attack actually stacks with the Bladelock Extra Attack, and the elixer.
Guess what still almost never happens. Guess what would happen even less if I wasn't Champion subclass.
Mind you optimizing effectiveness was never the actual goal for building that character. I just wanted to do cool stuff with the features gained from my initial narratively based class choice.
