You're right in that no amount of customization is going to matter in terms of easy content, particularly open world (solo) content. If things got unbalanced enough, it could affect stuff like normal trials and dungeons, but the existence of the duty finder makes it difficult to exclude or require certain classes or builds for day-to-day content. However, lack of balance in high-end content affects everyone, whether you participate in that content or not. Abilities must be balanced for that content or you're going to have serious problems with the endgame community, which is not good for the health of the game.