That's why it's great that Death Ward is useful against pretty much anything. As I said, if an evil wizard jumps you with 9th level spells with no warning right in the middle of your nap, your problems are much bigger than PWK anyway, so it makes sense to at least focus on a party who is at least combat-ready. And Death Ward absolutely deserves to be part of the standard combat-ready package. It lasts for eight hours, no concentration required, and it guarantees that you'll survive mind flayer brain-sucking, red dragon breath, Meteor Swarm, PWK, one banshee wail, etc. It's no panacea but what's not to like? If you have someone who can cast it without crippling their spellcasting for the rest of the day (i.e. multiple spell slots 4th level and up) you should probably cast it, more than once. IMO.
BTW, I remembered another mitigation for PWK: Revivify. Pretty sure that takes only one round to cast, which counters 9th level PWK in the action economy. If PWK had a save that would look even worse.
PWK wouldn't look so bad if you replaced the HP requirement entirely with a save. Save or die, straight up, no HP limit. You had mentioned raising the HP limit in conjunction with adding a save I'm not sure whether you considered going that far or of you were just going to raise it to 140 or so, which would still be meh.