Why don't people like to pick/use Tenacity over Determination?

[Kabooa]

While your theory is correct and relative to each previous step it is a steady increase of +25% eHP (and relative to the original it grows exponentially), the reason we call it "diminishing returns" has nothing to do with eHP; the core of the explanation was "you can't reach -100% damage taken stacking %-based cooldowns because of the calculation being multiplicative, not additive", i.e. 20% + 20% =/= 40% but 36%.

Apart from that, you mistyped the 3rd mitigation. That should be 48.8% mitigation (51.2% remaining) to give 195.31% effective HP; 51.2% mitigation would be 200%+ eHP.

Yeah, I mixed one up.

But that's also not what diminishing returns means. It means your effective gain is less. The effective gain is the same.

If it wasn't a geometric ratio, then you'd have increasing returns.

Right now it's neutral. Each gain is the proportional same as the prior - not diminishing.

For example

Stacking Direct Hit is diminishing return, even if no soft cap is applied.

1% DH is .25% average damage increase. At 10% DH you have a 2.5% increase of damage.

Our current baseline is [0], but we'll use [5] instead because it plays nice. 0 to anything is an infinite increase.

Every additional % of DH from the previous is 'less effective'.

5% -> 10%: "100% more chance"
10% -> 15%: "50% more chance"
15% -> 20%: "33% more chance"

Linear scaling improvement such as DH, Offensive Tenacity, and Determination have diminishing returns. That's before a softcap is applied.