The main issue with a 21 character slot limit is how restrictive it is for players.
At max, most players will have 23 if they do the insane monster collection page, and Zero’s Temple. That still leaves ... 20? classes that they just can’t play without deleting something else.
Take into account the 12 classes needed for link skills (even if you main one of them, you don’t want an empty slot) and then possible legion slots you want?
That basically requires you to gimp yourself in the mid to late game because of some ridiculously arbitrary restriction that doesn’t exist elsewhere, and if you want to switch mains, you have to delete a character that you’ve put time and love into. It’s absurd, and there’s no good argument for it imo.
I don't have any problem if getting more character slots is challenging, but it takes a lot of time to get all of them I think now there is no person on this server that has all character slots, so maybe they should focus on this topic