Don't care enough to argue with the people desperately gripping onto a minority of flame/Scourge-related NPCs or a single Azerite ability, but I'll chime in and say: I agree, fire DKs are a bad idea.
The class (much like everything else in-game nowadays) doesn't need more of its core identity undermined. Death Knights have a solid, coherent theme. There's no reason to dilute that for something that's extremely generic, to please a slim minority.