Gamers that don't like the horde mechanic will stay away and won't experience what else the game has to offer.
That doesn't mean that'll be a downfall. It simply means people who are tired of zombie hordes may avoid Days Gone. Like any other game where the material is either not to their liking or they've grown tired of it.
Plenty of people will still check it out, and with a userbase pushing 95 million by April, that is a lot of potential buyers within it. If it reviews well enough, it will definitely be a sales hit.