Imagine that a game had two types of classes:
Solo classes capable of completing all of the leveling content on their own, but unable to join any groups and therefore excluded from the endgame.
Group classes that are tuned to require balanced groups (the exact size is unimportant) to complete leveling content, but in return receive double exp/player/hour and are allowed to continue into the group-based endgame.
The balance issues that come with having to balance every class with a core solo PVE skillset and group utility would be gone. So would the challenges of providing solo and group leveling content, since both types of classes would be completing the same content. Meanwhile, the complaint that the solo leveling game does not prepare players to fill their roles in endgame groups would be gone.
Would It Work?
Someone who knows their history might be about to comment that the old school group games like EQ1 and FFXI actually did have some classes that could solo by charming or permakiting mobs. These classes were incidentally not so useful to groups, since their tactics were superfluous with a real tank and healer. If anything, I'd cite that as a proof of concept.
If developers implemented this intentionally, they could balance the leveling experience to be fun for everyone, rather than fun for one group (either soloers or groupers depending on the game) and annoying (grindy or trivial) for the other. There would be perfectly clear expectations so that players don't reach endgame and discover that their classes aren't wanted. Because the characters would be on the same server, group players could have solo class alts for their farming needs. Alternately, the game could use BOE endgame group dungeon loot as a way to encourage solo players to send their cash to group players for their upkeep costs.
The devs would technically miss out on the chance to try and convert a 95% solo, once/month potential group player like myself to the endgame raid gear grind. Then again, it's not like I'm currently able to do group content often enough for that to be an option that I would miss if it were gone. In any case, I don't think that's a huge portion of the market compared to the people who are impacted by the solo -> group playstyle shift at the end of solo-friendly games like WoW, EQ2, and LOTRO.
Solo players would be free to solo, group players would have team-mates who know what they're doing, and devs would save some time on balance and content addition. I'm sure there wouldd be new and different problems with this model, but it seems like it'd be worth considering.