1. You're not being punished, the level sync allows the content to remain somewhat relevant and prevents high level players from destroying the game for lower level players. Level sync is/was one of the most attractive features of this game for me as I have played many a game where high level players made playing a nightmare because they could totally overgear everything. Perhaps you don't remember the outcry when Atma first became a thing and parties of level 50 players (even level sync'd) in FATE were basically obliterating FATE before leveling characters could get a look in. Imagine how much worse that would have been without the level sync in place.
2. Your suggestion that people should learn their class in open world content before venturing into dungeons is complete crap. Apart from the self serving nature of the suggestion since it keeps those players still learning out of your self important path, it's also total nonsense, you can't learn your class properly playing open world content. Part of learning your class/job is learning your role in a party. That is something that can only be learned in dungeons.
But, you can't learn that in any content where overgeared players destroy the game balance - which is precisely the problem with the negligible ilevel sync already in place on the level 50 dungeons. There is no game balance. All that work of balancing content and jobs/classes is utterly wasted when overgeared players can ignore all mechanics, pull the entire dungeon and AOE the entire trash pack into the dirt. If all you want is speed runs and a gear dispenser, perhaps you ought to be playing GW2?
How would it be a waste. Level sync preserves the relevance of the content. Right now, for a player in end-game gear how much content left in the game presents anything but a trivial challenge? What % of the total game provides any level of challenge to your skill as a player? Level sync and ilvl sync are there to *try* to preserve what little challenge is left. It's not about you and your gear, it's not about making obtaining your new shiny gear feel like a waste. It's about making sure that the game content stays at least somewhat relevant. Right now, for anyone with gear from the current patch only content (non raid) from the current and prior patch really provides much in the way of a challenge. The rest can be run with minimal effort.
The giant waste here is the waste of decent content because of a total lack of game balance due to players vastly overgearing the content.