Did Kiera really deserve a happy ending? She may have been the protagonist, but she was a willing enforcer for an oppressive, totalitarian state, and it wasn't until the final season that she belatedly came to realize she'd been on the wrong side. She did some good in the present day, but she'd also done a lot of bad things in the future. And for most of the series, she was pursuing a very selfish goal, willing to preserve that dystopia and the suffering of millions just so she could get back to her own cozy family life. Yes, in the end, she belatedly chose to do the right thing and prevent the state from ever arising, but that doesn't totally wipe her slate clean. She was not the best version of Kiera Cameron, the version who deserved that happy ending. Dramatically, she had to pay a price for her bad choices. She succeeded in reuniting Kiera and her family, in creating a timeline where they were never separated and the dystopia never existed, but at the cost that she could never be a part of it, because she was a relic of a darker reality. That is the closure. That is the payoff, quite conclusively.