On a personal level I completely agree with you. However what one person finds boring another person finds fun. What one person finds easy to understand another person finds frusterating. From a marketing perspective it makes sense why you'd want to have the game very accessible even if it meant making everything extremely samey for balance purposes. On top of that when you have to create content like dungeons it becomes exponentially harder to balance for every player option available. As awesome as it is to fantasize about the possibility the reality is all this stuff takes money and time to do. Unfortunately neither is as infinite of a resource as we would like it to be for a developer.
That's why I think splitting it into seperate modes is a good idea. You can give them the same base content with some exceptions on either side. Solo side could get some unique duties and normal side can get things like raids. It would give people who want these extra options a place for them yet wouldn't destroy the balance of the game for those who like it as it is. That way you make as many people as happy as possible and, let's be honest, from a financial perspective that's what you want to do. Reworking huge chunks of the game is a good way to upset half the playerbase into dropping immediately especially when the game has been established as the way it is for many years now.
One thing that could be done, though, is allow "Solo Mode" builds into duties together unsynced. Basically a "Let's just all be OP" type of thing. It is still an MMO after all, and showing off builds can be a lot of fun. The entire point of solo is to not be concerned about balance in order to bring in unique ideas and player diversity. Unsynced has always been a way to simply curbstomp everything without care for balance. It's a natural fit.