Spammers and scammers already use domains as a disposable commodity creating them or using hacked ones for single campaigns and moving on. Part of filtering based on IPv4 is not only scarcity but accountability. When the owner of the netblock reassigns the ip and its already blacklisted it can create a problem for them and incentivize them to police their own network. Domains are also worse in that its easier to use fake information and be untraceable. its also understandably easier to get a response legal or otherwise from a co-location or isp than a domain registrar. Maybe ipv4 will always be preferred for email just because its more difficult/expensive and therefore less appealing for temporary malicious use.