For every +1 we can stack to crit range, the weaker Champion becomes because the relative increase get smaller. Going from 20 to 19+ for crits is a 100% increase. But from 19 to 18 is just 50%. 18 to 17 is 33,3%.
Champion might be the most boring subclass, but this is a really odd way of evaluating an ability. You might as well say HP, ability score, or spell slot increases are weak because each one is a smaller relative increase.
If we're calculating the dmg output a feature gives, we need to look at how much additional dmg it gives relative to not having it so we can compare it to other features. Let's do a really simple example:
A greatsword has 2d6 (7) dmg that is multiplied on a crit. On average, that is an increase of 0.35 dmg per attack you make with that weapon if you have a 5% chance to crit, AKA 7 extra dmg per 20 attacks. With 10% crit rate, you gain 0,7 dmg on average per attack (or 7 per 10 attacks), which is twice as much bonus dmg.
Compared to Battlemaster, how many attacks will it take before we go over the value of their superiority dice? Since superirity dice are only used up when they have an effect, we can assume 4.5 dmg + crit modifier. With 5% crit chance, that's 4.725 dmg per superiority dice and they have 4-6 er short rest (and they can become d10 instead of d8 for slightly more dmg, but let's stick with d8). With 4 dice, that's 18,9 extra dmg per short rest. That means it will on average take about 54 attacks per short rest for Champion to earn the same bonus dmg with a greatsword.
But if we start with a higher crit rate, which is not that hard to achieve in BG3 between equipment and elixirs, Champion no longer deals 100% more crit dmg. If we start with 19+ (18+ for Champions) they only deal 1.05 compared to 0.7 additional dmg per attack, which is still just 0.35 more on average. So the value of picking Champion compared to another subclass gets smaller and smaller the better your baseline crit chance is.
Again comparing to Battlemaster, it's now +0.35 dmg per attack for the Champion versus 4,95 dmg per superiority die. So it is going to take roughly 57 attacks to break even with 4 dice per shortrest. The higher the base critical chance is, the more attacks a Champion has to make to break even with a Battlemaster, because the Battlemaster also scales with crit chance.
But let's go all out with a Barbarian 9/Champion 3 Half-Orc with a greataxe. They deal +3d12 dmg on a critical hit and with Reckless Attack they have 19% chance to crit. Which is 3,705 dmg per attack. Swap out Champion with Battlemaster and we deal +3d12 on crits with a 9,75% crit (with Reckless Attack) for 1,90125 per attack. A difference of 1,80375. What if we increase the base crit to 10%? Now the Barbarian 9/Battlemaster 3 deals 3,705 dmg per attack in crits vs 5,41125. That's a difference of 1,70625, which means the relative bonus Champions got is now smaller. All the while, Battlemaster maneuvers slowly get stronger with each increse in crit chance. And this isn't comparing the value of the maneuvers themselves, nor the value of burst vs sustained dmg. Burst dmg is very valuable because the more actions you can deny the enemy the better.
At 19% crit rate (base 10% with advantage), Battlemasters do 5,355 per die. So 21,42 per short rest. That means the Champion breaks even around around 12-13 attacks.
Edit: Accidental math. It should be +3d12, not 4d12. Working on fixing the numbers. Edit: Updated.
This is the only build I can think of where Champion can be good, and it only works at max level because you need Reckless Attack and Brutal Critical from Barbarian (minimum 9 lvls) and you need to be a half-orc. So Champion is literally just a dip on another race/class combo that can actually make use of the incrased crit chance. And it doesn't really work as well with greatswords, since it doesn't increase your crit multiplier, just 1 additional dice last time I checked,so a greatsword crit would be +4d6 and not +6d6. Which complicates the comparison a lot because of all the unique items, where greatswords have some singers.
To apply a comparison to HP, spellslots or ability scores, we have to compare it to what else they could go towards. All HP stack, so it's just a categorically different comparison. But for spellslots, you don't want the spellslots for their own sake; you want them for the abilities they let you use. However, unlike a Champion's crit bonus, spellslots also consistently unlock stronger abilities to use them with and the ability to upcast spells you already have. The more spells and spell-affecting abilities you have, the stronger spellslots become. This is contrary to how Champion works, because the more you can stack your crit chance, the weaker Champion becomes relative to the competition. Diminishing returns 101.
And for ability scores, you can simply do the math for whether increasing them or taking a feat if better, including how various items in the game will affect it. Are you willing to rely on Elixirs for your str score? How many feats vs ability score increases do you actually need? Etc.
