MMO ≠ grouping. When people conflate MMOs with grouping, this is less an accurate reflection of what an MMO is and more a statement about someone's preferred way of playing an MMO. There's nothing wrong with this preference, of course, unless that person thinks that because they exclusively enjoy group content, then everyone else should be forced into grouping whether they enjoy it or not. Unfortunately, this is usually the case when someone is advancing some form of the "because it's an MMO" argument.
For me, the hallmark of overworld/open world content is that it can be enjoyed by an extremely diverse range of players; whereas, to my mind at least, instanced content is conducive to more narrowly-focused content that appeals to smaller subsets of the community. Now I guess whether Eureka is overworld content could be debated (though I think if it's not, then perhaps it should have been), but it certainly does not do a good job of catering to a diverse range of players—if that intention was even remotely present during its development.