To Be honest, I felt very disappoint for the change for Dark knight (DK) in Endwalker(EW). As for what I heard from others, The developer setting the dark knight direction to be more focus to damage mitigation with proper mana regen and less HP regen compare to other tank especially warrior (WAR) and paladin (PLD). WAR of course more focus to self healing with less mitigation, PLD more focus to party utility, self HP/Mana Regen and of course blocking attack, and Gunbreaker (GNB) more focus to dps.
However, for the change that they announced, Other tanks seem to have more mitigation than DK, which makes DK lose its position as one of the four tank available, not to mention all of them have more self healing. Even GNB, a dps tank had more self healing than DK.
Yes people may say DK had "The Blackest Night" (TBN), which is the best damage mitigation compared to other tank. However, this will not be a thing in EW since other tank had their own version of TBN. Even before EW, the only mitigation DK had as a tank that superior than other tank is TBN. Giving other tank their own TBN version will surely make DK loses its advantage.
Yes, people will say "but you can absorb damage equal to 25% of your HP, which makes 100% HP amount since TBN only had 15 second CD, combined with the new ability Oblation will become 35% every 15 second, which make up to 140%." No that's not true. Both are different type of mitigation, one as damage absorb and latter as damage taken reduction.
The reason why we use TBN is because we want it to break so that we could gain a charge of Edge of Darkness/Edge of shadow. Therefore maintaining dps since all the ability mentioned consume 3000MP. We wont pop TBN if we didn't assume the shield will break in time (TBN last 7 sec). Therefore, we only pop TBN when tank buster occur. Most of the dungeon boss normal attack won't break the shield in time (excluding those that had damage increase mechanics and damage taken increase debuff). The awkward situation is when you pop TBN to withstand boss attack, later on boss start performing tank buster and your TBN fade away (TBN still in CD), this became more difficult while fighting trial/raid boss where mechanics changing are harder to read.
DK dps is no quite decent compare to other tank, Most of the time is only on par or lesser than other tank. therefore the only thing tank has that is unique and superior is TBN. But this will not be a thing after EW release.