I wouldn't have an issue with it if it was the rule, I can imagine fluff to explain it satisfactorily, but I wouldn't change it for myself as I don't think it captures the scale of the units it does or doesn't work on...
For me though fluff-wise, I can see it carving through metal armour (an ogres belly plate or the armour on a empire chicken knight) as it generally the same metal as on infantry or cavalry... but I don't like it slicing through a humungous monster's scaly skin or regen save... The big gribblies suffer enough, I think a small scratch from a nasty skulker's attack shouldn't ignore a a regen save of a monstrous chimera..
So then I'd end up keeping the original rule as it's cleaner because I think the suggested change doesn't have enough nuance and I'd be loathe though to have 1 rule with a series of forgettable caveats for edge cases... Killing blow works like this in case A, it's different in case B and doesn't apply in case C.
Trying to figure out how to deal with armor for these puny little goth elves. Open to suggestions!
Use chaff to get the heavy armored stuff into combat with your Executioners or divert it harmlessly. Even if KB doesn't work on a target, massed S6 attacks is your best unit for the job.
Part of this is using chaff to stretch out your deployment and force your opponent to drop his big units first so you can counter with your own deployment. DE is a maneuver army, almost as much as WE. You can't just march straight across the board and smash everything in your path like some other armies can. You can crush most core units easily enough, but don't let that get you overconfident when dealing with heavily armored elites.
Magic can also help. Lore of Metal, obviously. Any lore with strength-boosting augments, like beasts. Yes, Mindrazor is the best, and a horde of Witches with Mindrazor can delete whole units in one round, but don't count on getting that miracle-worker off when you really need it.
Characters and magic items can also help; DiscoQing covered those options well enough.
Dark elves have some of the best anti cannon tools available...
Buckets of fast cav Harpies GW wielding Scouts Magic missiles coming out of the wazzoo 1+ 3++ flying characters
A hero on a fast cav horse with 1+ armour and (asf) Lance is 98pts...
Which is true, but it's hard to stop two or three cannon all at once, AND win the chaff battle so your big blocks can do their job. It's often better to leave the monsters at home and rely on blocks to do what blocks do, and use chaff to get the block matchups you want.