An attacker can send messages from my account before I even know that my key is compromised.

When my account is compromised, both the attacker and I can post from my account, as long as your idea isn't implemented.

In such a scenario I will post that my account is compromised and provide *evidence* via other social media accounts. The attacker can't do that.

I could also prepare in advance by setting up 1-2 backup nostr accounts that only I have access to, which are unlikely to be compromised at the same time.

Your idea would prevent me from posting from my account, and that is very dangerous.