There is an alternate universe where this month a fantastic Inhumans movie dropped and the franchise suddenly achieved a Black Panther-level shot in the arm. That didn't happen, but the growth they've seen this decade is unprecedented and they've firmly marked themselves as a real part of the MU instead of FF supporting characters or kooky cult characters, with greater prominence in other media like animation, games, and merch. I would not call the Inhumans push a failure in any way. The same way Marvel played into illogical fan paranoia for Death of X, they played into the TV show reception for Death of I, but it's just another chapter in their story instead of some statement about the brand. We'll see how it plays out soon.
But they were never a threat to the X-Men, and were never supposed to be.