I disagree.
In DAO the mage was the most powerful class in the game and yet the other 2 classes remained popular with the players.
In Dragon's Dogma the Assassin was the most powerful class in the game and yet every class was popular with the players.
In DnD the caster classes are potentially the most powerful and yet many non caster classes remain popular.
I imagine this has to do with the fact that the people who create these characters are ROLE PLAYING.
I disagree with most of that.
My best Wardens are always rogues. A well kitted out arcane warrior couldn't match my rogues by end-game, especially with their terrible melee damage and insane fatigue and mana costs greatly limiting their casting. And if not an arcane warrior, the comparison is even more in favor of rogues due to mages that aren't arcane warriors being rather weak in comparison. Not to mention most of the best offensive magic is more dangerous to your own party than it is to the enemy thanks to friendly fire... Assuming you play on harder difficulties anyway.
Mystic knights are the most powerful class in DD, not assassins. Assassins only had really overpowered augments, but the class wasn't anything special. Just a glass cannon with high damage and weak durability, with masterful kill as the only truly awesome attack they have thanks to it blocking just about every physical attack in the game. But mystic knights are the most durable class in the game on top of having the highest damage per second out of any other class. A mystic knight can kill the devilfire grove drake in less than a minute by level 30... Good luck getting an assassin to pull that off. Also physical damage gets greatly eclipsed by magic and elemental damage as the game goes on. This paragraph isn't my opinion, it's fact. Mystic knights are the most overpowered class, period. And this is coming from me, someone who actually prefers assassin Arisen's and has done more playthroughs as that class than as mystic knights. But I don't delude myself about which is clearly superior.
So you're wrong from my perspective in the first statement about DAO, and absolutely unquestionably wrong on the second one about DD.