Even if we agree that the game is moving towards symmetry I'm not sure how that argues for imbalance because symmetry is a type of balance. Since the community as a whole seems intent on building solo capable characters and appears to have an inexhaustible set of reasons for why the game must accommodate soloists I don't see what the problem is.
What I find intriguing is that the same Wiki article that discussed game balance also mentioned difficulty level. Thrudh mentions building to solo epic hard while grouping for epic elite. If that in fact describes the game then, once more, I would argue that the game is balanced. And, I would contend that it succeeds in creating the right environment for cooperative success provided that the general point of view remains that nearly any combination of characters can complete EE.
Now, it has been a very long time since I've seen anyone arguing that a group cannot succeed if X class is present. And, with respect to the Shapley value, it is pretty irrelevant if there are no combinations that cannot succeed.
So, Thrudh's criteria of build to solo EH and group EE seems to suggest that balance does exist.![]()