Parry...

[MythToken]

Um no to everything you just said in the beginning of your post, nothing in this game has diminishing returns. Every single test ever run confirms this. The exception being Speed, and ACC for different reasons.
Crit being the only stat that might have an argument for diminishing returns, even though I still have not heard a good one.

But I do agree with your mitigation argument. It is easier to get than DPS, and why this is even an argument is beyond me. Tanks are here to mitigate damage.

If you want to add dps and speed run through things, then just bring an extra dps and 1 less tank. Thats the only real way that actually works.