Death Knights didn't kill Bear tanks in world of Warcraft, and Dark Knights won't kill warriors in FFXIV. Just because DKs will have superficial similarities to how warriors play, doesn't mean it'll feel the same. There will always be players who prefer warriors. All this extra class will ultimately do, is add variety for people who are already tanking or might like to try it but aren't sold on Paladins or Warriors. As for being denied raid spots because of your choice of tank class, that actually doesn't happen very often in my experience. Mostly people complain on forums that it could happen, because we all know that min/max harcore raiders set the standards for how gaming populaces behave. (Wait, no they don't.)
Someone somewhere will always find an excuse to look down on others over petty minutia. Screw 'em, Pinkie. Play what you like.