Don't worry, TR will have long legs, the exclusivity deal and XB1 not being the top selling platform has probably caused at least two thirds of the gamers to wait for other versions, it'll get another sales boost later in the month when PC gamers get to play it with even better graphics and nice framerates, and later in the year it's PS4 owners turn to get a taste of one of the best games from 2015 in the assumed Definitive Edition.
Plus, Horizon is a whole other game, it won't sell poorly just because it has a girl in the lead, I'm more worried about the actual gameplay, the concept sounds awesome but GG's games has mostly wowed me with the visuals.