So, I think you have a solid issue in mind but you're stabbing at it the wrong way. The leveling process in every RPG is best used as a tool to educate your players on how to play. That's why in multi-button MMORPGs like WoW, Rift, Wakfu and XIV you unlock abilities at certain milestones. Unfortunately, not everyone understands this as the intent. I have a friend who's wife attempted to pick up WoW and got all the way to 54 thinking that white damage (referred to as auto attack elsewhere) was the fastest way to single target.
One must keep in mind that this is, ultimately, a social game; obviously more in the tradition of Cards Against Humanity or Poker rather than Farmville or FFRK, the primary engagement is supposed to be social. The developers of this type of game usually expect players to involve some level of research and discovery to get factored into the learning process.
In short, we are supposed to teach these people.
Of course, the difference between this and, for example, Rift is that the devs attempted to add a sort of "shallow end" to this educational curve by locking most of the really technical spells and all of the high-level content behind a quest. This is basically intended to tell us we need to finish mastering one skill before the next gets unlocked. It is supposed to feel like they're saying "before we give you desert we want to make sure you've eaten your spinach".
This may require a few tweaks in the system design. Maybe they should lock even the low level dungeon behind the class quest of appropriate level. For example, maybe they could say "If you want to run Satasha as a Lancer you must first complete the lvl 15 Lancer quest. They do it already for 8 mans with guildhests, I don't think it would be that big a stretch since the infrastructure is already in place.
Fortunately, I am certain the social aspects of this issue are largely soluble with a few minor changes in the mindset of the community. I think a few minor tweaks --- such as DPS exclusive dungeon commendations --- would go a long way to helping us understand that other players aren't just stepping stones to the next loot piece. I would certainly stick around to Com the guy who showed that Summoner how to use something other than Ruin and that would definitely encourage her to mentor any other Summoners she runs into. Very often all it takes is a pat on the back for your hard work to encourage you to improve.
Remember, negative reinforcement is cumulative, positive reinforcement is exponential.