I disagree with the diagnosis (although at least you tried to answer the question "why"): I don't think people is saying it's mobs hp that is causing those builds (there are more than it seems, but probably you refer to all succesful builds that are effective while ranging in whatever form) to be the only one viable -> no, it's the damage output they do that is too unmanageable for melees to stand and therefore contribute with their dps (which is perfectly fine) and enjoy the fun. There is a consensus. However, I have to say that some melee builds can still deal with that with some proper healing backup... Let's leave it there...
So you do not want to make your character more powerful by taking 1 class split or 2 that is available to you and therefore if someone is doing it it is overpowered? When an option is available to everyone, that is not overpowered. Another thing is whether it becomes gamebreaking. 2 monk breaks the game? why? Nonsense. Multiclassing should always be rewarding. why should a fighter 20 be as powerful as a 16/2/2 or 12/2/2? You even go further as to say it should be more powerful? Why? Multiclassing options are there, always have been, take more planning, and open so many possibilities. Sure, you don't want to multiclass, don't, and wait for someone to join the group to do some specific task you could have performed with some extra planning investment. Capstones are already good enough, they are a freebie that actually is debatable. "Give us an incentive to stay pure". Still, why? If there is an item that goes in slot A and makes you way more effective, you can use it or not, that is your option, at the cost of using the slot, why should any other item have a similar level of power to your build. Some will be sinergetic and some won't. That's ok and fun. Well you could argue the same with PLs. Give us an incentive to stay single live!! Single Life toons should be as effective as completionists (free that feat already, come on!), "give as a reward for not having TRd". Seems like a twisted logic but it is not. The option is there and is available to you. You cannot choose to not take the available tools/options that the game provides to be more powerful/effective/efficient and then ask for your build to be as effective as the one that optimized all.
Yes, gamebreaking stuff needs to be changed. EiN is not what once was, quivering palm seems to have reach some kind of middle point, etc. etc. If there is an ability that kills all mobs every 10 seconds at no cost, sure go ahead change it, no problem. Some people seem to have a fixation, I don't know where that comes from, with pure builds. DDO is not about pure builds and then there was a mistake in design that allowed you to mix 3 classes. Multiclass is the bread and butter of DDO.
Along this thread something has been identified: melee gets too much damage on EE. Fine, let's see what can be done, but that has to be balanced towards the mentality that melee has tools available for versatility.
Solo/party. Some hatred to solo play also among the thread. Some people has fun soloing stuff and some people hates solo play and understands the game as a party where there are definite roles. Well, this game is fantastic at pleasing both possibilities. For me, I like partying a lot, but soloing EE is a very nice challenge, and I consider my toon is correctly built (for me, of course) when it allows me to solo that content (with the obvious exceptions). And yes there was a time when a Barbarian could go out there and solo the hardest content. We had a legendary angry Axe wielder out there that broke many missconceptions...
Funnily enough, another legend around here, MrCow, predicted the possible "paradigm shift" that ranged use could bring to the game (when BAB would increase enough and some other variables took place) some years ago, when ranged was still laughable.