Key rotation was much, much more important when ciphersuites without forward secrecy were widely used, because then old server private keys could be used directly to read old traffic. Now that forward secrecy is common and routine, the benefit to key rotation is less obvious (although I think it still exists). We greatly reduced that benefit for the non-forward secret ciphersuite users by not making Certbot delete the old private keys. 
I guess our threat modeling wasn't explicit enough.
Anyway, I'm still happy that Certbot's default is to rotate keys—but I'm not confident enough of the concrete benefits of that behavior to try to pressure people not to use --reuse-key!