Spamming users with warnings that 99.9999% of the time will not be understandable let alone actionable is a quick way to get another SSL warning dialog scenario.
I really hope it's not a new set of keys every time Whatsapp updates...
You could argue (and I have done) that a key change event should be surfaced in the UI as "Your contact has switched to a new $MODEL_NAME phone" so if it occurs in the middle of a conversation or you happen to know they really haven't upgraded, it can lead people to tap it and learn more. An alert like "Contact key has changed" would be meaningless. But it's also the case that notifications that seem useless to people rapidly get phased out, and unfortunately any notification that boils down to "you MIGHT be being attacked by your own government but PROBABLY your friend just dropped their phone" is a non starter.