"Successful" being a very important bit here.
WildStar was built by a dev team with a philosophy that "people want an MMO that's hardcore again, with raid content that requires a ton of cooperation and coordination to get through, and main story locked behind skill checks." And as much as I love the WildStar devs as people, I also think they were proven very wrong. Because while it was really neat content... a huge amount of the playerbase got to like level 25 or whatever, hit a skill wall, got frustrated with the inability to progress, and left; sure, some were motivated to 'git gud', but many others were motivated to 'git back to whatever other MMO they used to be playing'.
The game lost so many players—and so much money—that they first went free-to-play with RMT, and then just entirely were shut down. Because no matter how vocal the "hardcore" players are, they're far less numerous than the casual ones... and far less able to financially support an ongoing game wholly on their own.
Now, that "cater specifically to the hardcore playerbase" philosophy wasn't the only factor in WildStar's decline and demise, but from what I observed it was certainly a big one.
And as much as I've come to love savage and such since getting into it in this game, and look at that sort of content as a yardstick to measure my own improvement against... I also have many friends who are not into it, and who would be heartbroken to see lore locked behind it. And if it was required as part of the main scenario—like the skill check in WildStar turned out to be—they'd probably go play something else instead.
This game could definitely stand to tweak the balance in a lot of places, but if you tweak things too far you will chase away the playerbase.