But those points hold nothing against Greninja or any realistic fast, strong Pokemon, for that matter. You will (hopefully) never face a Pokemon that is bulky, strong on both offenses, and fast. Yes Greninja is frail but it's something I see (sorry if I didn't get this across) as something expected from a Pokemon of its kind and isn't what keeps it from being a monster because, again, you won't see an omnipotent Pokemon in OU.You literally just contradicted your point when you said with that last sentence.
You literally listed the negatives of Gren. Low defenses on gren is an argument because that is what stops him from being a true monster.
So yeah this thing is frail, so are most fast, hard hitting Pokemon but that doesn't stop it from being broken. This thing can hit everything Super Effectively and with STAB before they can hit back. Mega Mawile was slow but that didn't stop it from being broken.