I'm kind of tired of the idea of "sandbox will solve everything". There's been other sandboxes since Eve. There's others now. Many of them have struggled. Ones like Wizardy Online barely lasted long enough for people to realize it existed.
Variety should exist and I agree that 5 100lb gorillas is healthier than 1 500lb gorilla, and that's sort of where things are going right now. Eve still exists, WoW persists, SWTOR is healthy in audience and money, FF14 ARR did better than I ever expected, and GW2 is the biggest MMO success in awhile. The days of the monolith are ending.
However, there may be actual solid reasons the themepark exceeded over the sandbox. It may be that a sandbox of millions of players together doesn't work. "There shouldn't be a reason concepts like Minecraft can't work on a massive scale." Actually, there is. Think of your school playground. Did all the kids play together? No? Did they sort of spread out themselves? Into their own groups? There's a reason: everyone has to buy into what the group is doing or it falls apart. If Sally comes into your pretend fort and starts turning the guns into talking bananas, everyone else's little game falls apart.
Minecraft works because it's a select group of acquaintances having imaginary adventures in their own little circle of play. Once you invite everyone into that circle of play, then things start getting messy. Everyone blowing up, building, and rebuilding the same limited amount of space will be like the history of mankind happening at a internet consumption pace. Rome will burn ten times a day.
That doesn't mean I don't want more player creativity within the MMO genre. I like player housing and open environments to do your own sort of thing. A good crafting/trade system is very interesting on a massive scale. However, not all things work on a massive scale, and that's an even dicier situation when factor in how toxic the online gaming community has become. Not everyone wants to go log into their game to hang out with hot-pocket fed psychopaths.
I also have issue with how sandbox expects the player to do all the content creation. That's more a social issue with the design though. So is my concern that this is only feeding into a bad consumption cycle.