We’ve always known that fel is more powerful than arcane, all things being equal. The downside to that power is that it eventually destroys everything around it. The orcs were able to steamroll the arcane-talented draenei once Gul’dan spread fel use amongst the Horde. But the consequences of that much fel magic was that they rendered Draenor nearly uninhabitable. If they hadn’t invaded Azeroth they would’ve gone extinct. There are risks to using the arcane too, but nowhere near the risks of using fel.
As for your generic mage losing to your generic warlock, it all depends on skill. If a warlock begins summoning a massive infernal but takes an arcane missile to the head before he’s finished, it doesn’t really matter that his fel magic is technically stronger than the mage’s arcane magic.
Now, that’s different on the scale of the Titans. Arcane magic is literally their lifeblood. They’re beings of the arcane. On that scale, with a corrupted Sargeras who at that point was basically a cosmic volcano of fel, then yeah, they’re going to lose. He didn’t one shot them though. They put up a fight, but in the end Sargeras wore them down due to his new fel powers. On the scale of individual demons? I’m pretty sure any Titan could just step on them and they’d be obliterated, considering Aman’Thul killed Y’shaarj by picking him off Azeroth like a tick and literally squishing him in his hand.