To everyone who says you can do everything on one character, you're mistaken.

Once you hit 50 you're restricted to gearing one class with myth tomes provided you want to make good use of your tomes, and you're locked out of coil once you've run it on one class.

If this game we're like others where there were no armory system and you just simply made alts, none of those restrictions would exist. You could run all endgame content with the class of your choice, and optimally gear said class.

So no, you can't do everything with a single character. Just most of it. There have been quite a few design choices that are baffling. They make a payment plan for alts, and then discourage against their use by making it impossible for you to utilize your assets between characters without the help of a third party. Then they implement an armory system that allows for all classes and then restrict your ability to optimally play all classes but one.