The TLD ".invalid" already has the required
property, and is registered with IANA as being guaranteed never to
resolve to anything.
that doesn't do a thing to keep the root servers from getting hit.
It does if user agents that receive ill-considered requests to send mail
to such addresses recognize that particular case and don't waste time
looking it up.
If we were talking about a completely new protocol, I might agree with you.
But Usenet and email are both widely deployed, and it's not reasonable
to expect the installed base of Usenet and email software to change to
avoid the impact on the root servers.
(for the same reason, the .local TLD as used by a certain poorly designed
local name lookup system is also a bad idea.)