My argument is that no one knows the in's and out's, not in such a changing game. Even if you were to know, it's all a matter of perspective. Playing solo is different than playing in a group. Grinding experience is different than defeating bosses. Training is invalid when used as a proposition to encourage new players to stay in the game when there is no one around their level to train with and higher level players are barred from legitimately playing with them.
On the contrary, you have a new influx of "noobs" that don't even want to play, let alone "learn how to play" because they can't even find people to play with. Even if they were to play, they'd just be soloing leves. How does that help in a party setting and how does that help with boss strategy.
Answer: It doesn't.
The question isn't "How do we incentivize players to learn how to play?" it's "How do we incentivize players to play with one another?". Well, at least if you're playing an MMO. It's not really a requirement for single player RPGs.
Varying challenges require varying tactics, thus "inexperience" changes it's definition based on context. We should not be controlling the context to the point that we are shedding new players and subscriptions. We should be opening up the gates to the theme park and sharing our experiences with new players.
Level sync does just that. The positives far outweigh the negatives.
Your concept of earning is very subjective. I don't really need to apply earning as an important factor in the enjoyment of a video game. It can be a part of it, but in a theme park game it's better left subjective. Some may prefer the pursuit of gear, boss fights, exploration, levels, or companionship. Not all of them need to be mutually exclusive but they can be if so chosen.
I throw my bets on the side of companionship for the case of creating a long surviving MMO. Otherwise just make a solid single player game.