Really don't see what the problem is with Dark Knight, staying alive....

Paladin, had 0 sustain till this Xpac, and remained just fine every new Xpac release. It had 2 mitigation cooldowns, and Clemency isn't sustain, it's a GCD cure that kills it's DPS.

Dark Knight, still has TBN, abuse it, use Arms Length and Reprisal. Literally been doing 1st EX trial with 1-3x stacks and staying alive just fine having aggro.

It's nothing to do with how good the job is, it's how you use the job to operate it, gear carries you, not the job unless it is actually broken.