First off, everyone that tells you to stack mitigation stats already knows about diminishing returns. It's not something that escapes our notice. In fact, it's one of the biggest things that has to be considered. Of course, you're also vastly overstating the impact of diminishing returns since it all operates on percentage decreases to damage taken. A 1% increase in defense is worse less when you have the base 5% defense chance than when you have a 30% defense chance (the first is a 1.1% decrease in damage taken and the second is a 1.5% decrease in damage taken; it gets even *bigger* when you get to the higher numbers with Shield and Absorb where 1% more shield at 65% shield chance equates to a 2.9% decrease in unshielded attacks). Diminishing returns doesn't exist to encourage people to stack Endurance; it exists to offset the progressive improvements in performance based on decreasing percentages of incoming damage.
So many people don't understand this about tanking stats