what is wasted? IPs. not like IP addresses themself were not an actifical limit - but let set aside this for now. if i setup an MX, it still needs to be on port 25, because the vast majority of MTAs are not capable of service discovery; and there is no Host header or SNI/ECH for SMTP, so for each mail.DOMAIN.TLD i have to buy an IP, just to put them over a single multi-domain postfix/exim in order to please those MTAs who are requisiting the HELO name to match to the MX's reverse record.

how wonderful a world-wide adoption of rfc 6186 would be to able to put multi-domain mail server on a single IP.