I fully agree with you. I am a player that loves to do every quest and doesn't care if I am a few levels higher that the end boss.
The other way I can imagine that made them implement a level cap is that once they gave the end boss to a certain level, they didn't want to "force" players to do all the quests to reach a minimum level at which they can "beat the boss", enabling lazy players, who do only half the content of the game (the main story), to still beat him while on the phone eating a pizza.
For me this is absurd. Why would you buy a good game and do only half the content?
So Dev should make Boss fight aligned with someone who completed 90% of the game. Not 75% or whatever as it is now, with no level cap so completing the game rewards you with a easier end game. Or, you could enable an option in the setting that auto aligned the end boss with your level.
I reached level cap when arriving at twin elms by the way. I havn't finished story yet, but I guess (well I hope) I still have some play time before me.