All you say is that if Fairy-types don't get matched with good secondary types they won't be anything special. And this is true for any good defensive typing we have. What matters is that Fairy-type is an inherently good defensive typing for the current OU metagame. If we will get Fairy-types with good stats, moves, abilities, and secondary typing to make this typing worth it is another discussion, but you saying that Fairy's defensive abilities are being overestimated is both wrong and without any reasoning.
Gen 2: Introducing Dark Type. Immune to the best type in the game. No common weaknesses. Super effective on the best type. Clearly amazing. Yet only 2 Dark pokemon are Gen 2 OU, and neither runs dark type moves.
The problem with your argument is the bolded part above. Gen 6 is not gen 5. Sure Fairy is an awesome defensive type when nothing runs Poison moves and only like two things run Steel moves. But when Fairy exists, this won't be the case. Fighting used to be a crap offensive type. Now its one of the best. Rock used to be an awesome defensive type, now its one of the worst. You can't just say that because this is how it is, then this is how it will be. Poison and Steel got offensive buffs, and practically everything has a move of one of these types to run. I just think it is foolish to make such assumptions of similarities between gen 5 and gen 6.