The gods have been doing this for an impossibly long time and a very large number of worlds. This isn't quite infinity, but you can see it from here - so if it was possible for established gods to die, I feel like the pantheons should've been trimmed down a lot by now. Agreeing to the bet might've been a uniquely poor decision on Hel's part, but surely there would have been worlds with strange circumstances before. Have the Northerners never ended up as (say) pacifists who refuse to worship Tyr? We know Odin took a bad hit during the previous world, but he's also the most powerful god by a long shot, so he's got a long way to fall. What happens if you end up as a marginal god in several consecutive worlds?