That assumes Stealth Rock which isn't a given, especially when we're talking leads, and it also assumes Dragonite doesn't flinch you. Take away those assumptions and even a crit won't give you the KO.
My point is that Noivern can sweep a team, sure, but it requires that your team fail to fit some extremely broad demands you should probably fit to anyway. I don't consider it broken, or even all that great really.
If everything is at best a check, and the checks aren't very good at their job, and you're arguing that Dragonite is very definitely not broken... this holds together because?
I've not played RU, but it's ridiculous to claim that nothing hard-counters Charizard in OU. Just off the top of my head are Aerodactyl and Heatran.
And is Mega Aggron viable as anything other than a counter to these?
Noivern dies in one hit from Azumarill, but not Dragonite. (And my position is that Noivern is powerful but fine whereas I'm not so convinced Dragonite is balanced) Dragonite can also just use Earthquake for a guaranteed 2HKO instead. Even Azumarill coming in off a KO is not certain that it can scare off, let alone KO, Dragonite, when we're talking a build specialized to counter Dragonite.
I focused on Azumarill because you describe it as checking 'all' Extreme Speed users, which it full stop does not. I could've been really obnoxious and pointed out how easily Pikachu with Light Ball or Raichu can tear it in half. I focused on Unaware Clefable because it's one of the genuinely best answers to Dragonite -which means the fact that it can't actually be counted on stands out all the more.
The other counters you listed you provided little description and they all have problems. Scarfed Gardevoir will not OHKO Dragonite and will be OHKOed in turn by Iron Head, Durant is not a great Pokemon in general (And if it somehow proved a counter Dragonite would just run Fire Punch more often), and Cobalion is vulnerable to the standard Earthquake, is not that tough, and is not that hard hitting.
Most painfully they all run into the problem that to even resemble a counter requires you perform a switch-in while they get a free move, or sacrifice a Pokemon to allow them in freely. If Dragonite can get a 100% certain KO before you get the opportunity to force it out (Because you are forced to sacrifice to get that switch happening), and this keeps repeating, it's doing appalling damage to your team without dying.
In what universe is Lucario a counter or check to any of let alone all of Dragonite, Zygarde, or Noivern?