Having a subscription is low on the list of reasons Wildstar flopped. It was also buggy as shit for an absurdly long time, its raid attunement process was a tedious grind and its marketing campaign was the worst sort of cringe, cupcake.
In my opinion, Wildstar failed because it tried to be a hardcore raiding MMO in 2014 (a good half decade after that sort of thing went rancid), built entirely on the premise that the kids who haunt gaming forums and bitch for the good old days of Vanilla WoW could support the game by themselves. It failed to understand that those kids are now thirty-somethings with adult responsibilities and rose-tinted nostalgia goggles with coke-bottle lenses welded to their faces. They don't miss the clusterfuck that Vanilla WoW entailed, they miss being junior high / high school / college kids who had the time and energy and freedom to stay up until 4am goofing off slaying virtual dragons with their buddies. Carbine listened to that incessant whining and didn't understand that they represent a dead niche within a niche and it bit them squarely in the ass. And the demographic for that sort of thing has moved on to MOBAs, Call of Duty and Overwatch.