I mean, we're currently dealing with the fallout from an apocalypse. Taking out a major character is something they're generally very cautious about, though I'd be fine with them doing that where it makes sense, and Thancred had one of those moments that they avoided it. Heck they nullified Zenos' death and made his whole touching end-scene meaningless. Honestly, if they explored a zombie apocalypse scenario then I don't think it'd make a difference. And we had the opportunity to try and stop a character (though not a major one) from turning and had the whole tragedy to go with it when we failed.

But zombie apocalypses and the undead is something that's been done to death (pun intended) and if anything, something like Sin Eaters, like they did here, for an apocalypse scenario was fresh. At least, it's not something I'd be behind, especially as it'd be an arbitrary direction to take that doesn't seem to relate to any threads the story has going. And we've already had our own undead story arc, as rightly highlighted by the post above mine, and it was a really good one that started out in early game, got its own HM dungeon and expanded into PoTD.