Why do people think that Hex is so great? It's a first level spell that's very hard to maintain out of the combat as when someone dies I think you have to transfer the hex? Unless I've misread it I'd rather save my Concentration for Silent Image and the like and my combat spells for things more like Evard's black Tentacles.
Note the wording:
(emphasis added)If the target drops to 0 hit points before this spell ends, you can use a bonus action on a subsequent turn of yours to curse a new creature.
You don't need to reapply the Hex right away, as long you don't lose your concentration or let your cursed target escape, you can reapply to a new target in the next combat before the time period ends.
It does become less useful at higher levels because, the instant damage you can do via Hellish Rebuke or Blight starts to require a significant amount of successful hits to balance out the damage.
And although Hex damage doesn't scale with level, even at 9th level it only takes about 10 hits using Hex before Hex (on average) out damages any other Warlock spell.