I think when people discuss wanting a sub it is for the base game. The sub is going to have little conveniences attached as far as we know, so I consider it part of the convenience side of the cash shop
AoC is not a MMO right now, it is a battle royal IIRC. Pretty sure they also sold land/houses etc in their kickstarter. Also, I would not really call them a shining example at this point haha
If you can find a successful mmo with JUST cosmetics, I would be mighty impressed and say that is the exception and not the norm.
The problem with tying yourself to a bad financial model from the get go is you are shooting your own foot. Release is far from the end of the road, stale content will drive people away much faster than a bank space or skill potion will. The thing about convenience items is they give the devs some slack to develop for the actual game, as they are recurrent purchases. Cosmetics do take time, would you rather have a new NPCs art mapped out, or some fluffy wings?
I have come across games where the cash grab was just as real towards cosmetics, and the game grew stale due to lack of content.