You're confusing complexity with just being trash. Arcane isn't unpopular because it's hard to play, it's unpopular because it's just a badly designed spec for PvP. It has extremely limited CC to blanket its burst, unlike Destruction warlocks or Frost mages and at the same time relies on cast times to deal damage, which basically means their opponent has at least 3 different ways of negating their burst at any given time. But even assuming that the arcane mage can get the distance to even start bursting is a stretch because most melee classes in this game have reliable ways of getting on top of them and making them do 0 damage and just shredding them through their terrible survivability until they're forced to Iceblock and die 5 secs after it ends.
If cringe was an emotion, you'd be right. I don't know how to explain this to you in a more potato way than I and other people already have. If DK and priest are at the same skill and gear level and DK has a large advantage over the priest that means whenever that DK and priest fight, the DK will win every single time because he has a large advantage even when all other factors are equal, which is the definition of a counter. In order for the priest to start winning reliably, he would need to create an advantage elsewhere, by fighting significantly less skilled or geared DKs, which is why the only thing you're gonna get are pitiful looks when you're trying to brag about how good you are vs DKs. The only thing that tells us is that you're fighting either bad or undergeared DKs or just flat out lying to make your tiny pee pee bigger.
You either admit that DKs have a large advantage over Spriests and thus inherently counter them in a 1v1 fight on equal terms, meaning the only way you've been winning is by fighting scrub DKs or you completely deny that DKs have any substantial advantage over Spriests and that's how you can reliably win 1v1 vs equally skilled DKs. Claiming that DKs have a large advantage and somehow you're still winning vs equally skilled DKs just makes you look like a cringy plonker trying to look special.