I think it's possible and will probably happen at some point, but certain changes must be made. First, you have to realize that a mainstream dynamic MMO like UO or SWG is no longer possible. The mainstream audience is too short spanned and casual for that now. So anybody making that kind of game has to aim for a niche audience of more sophisticated gamers. That's not a problem though, there are more than enough out there to support it.
The future great MMOs have to be sandbox and dynamic. It's theoretically possible that another themepark WoW will come along, but given the level of polish and content required, and the expectations of modern gamers, I wouldn't hold my breath. But it's much easier to make a good sandbox MMO, because you don't need to create a ton of content for it, you just need to design good underlying systems, which is more feasible for a smaller team.
One thing that would really help is cheaper third party tools and software. If you can license a third party engine, a third party AI system, a third party networking solution, and so on, that would allow the team to focus on gameplay design and implementation, and level the playing field for smaller companies.
You also have to learn from the mistakes of the MMOs in the past. There are tons of them in the graveyard now. UO, SWG, Eve Online, Roma Victor, Mortal Online, Darkfall Online, Ryzom, Xsyon, Salem, and others. You can't play around with half-assed releases. An MMO gets its critical density at launch, that's when it will either get enough people to exist, or fizzle out. So you have to have a playable/enjoyable state by the time you launch it. You also have to have a healthy PvE even in a dynamic sandbox game. Players need something to do when not participating in PvP or player politics/economy. You need well designed enjoyable game-play, so you don't spend your time grinding. Grinding is not a must for MMOs, it is simply a cop-out for lazy developers who don't want to design actual systems. I will give a simple example: mobs are braindead in MMOs, so if you need to make the player hunt, the only way to have them pre-occupied with it for a reasonable amount of time is to have them kill 400 mobs. So you make the mobs drop some turd, once for every 40 mobs, and require the player to get 10 turds. Hence grinding. Now imagine an actual gameplay system, where mobs have real AI, and run from the player, camouflage themselves, ambush/attack, hide in caves, etc. Now, you can just have the player hunt one mob, which might take 2 hours, but will be enjoyable and not be grinding. Every system has to be designed like this in a good game.