Nobody minded that all the T10 sets were similar thematically (based on Scourge units).
There's more similar themes here-- All the T16 gear is based on Titans.
Also: Death Knights will never have another good set, because Death Knights entail a very specific bit of lore. "Rogue" or "Warrior" is a very generic archetype that is found in every culture on every planet, but it's consistently more and more difficult to find relevant lore for Death Knights just because of what they are, and thus difficult to make a good set. Death Knights will always be simply wearing the bones of their enemies for the rest of forever because there's no more Scourge to make kickass Scourge Battleplate from.
Monks will start having this problem in 6.0. Calling it now.