So the entire world has to get the capability, then everyone can exchange email with the new protocol. Blocking spam was not one of its original design goals, and so it doesn't have authentication. While there are some efforts to add authentication to the protocol, there is still the problem of deployment, which will take years, and then the problem of distributing authentication. These are hard problems, and may be technically or practically impossible to solve. The demands by antispammers to completely suppress spam are unrealistic.
Relays are necessary in some circumstances .
Not everyone needs a relay. In fact, relatively few sites need relays that are not behind a firewall. However, if one outsources, then services that might ordinarilly be done behind a firewall need to be done with general internet connectivity. One such case is roaming consultants, and remotely located machines that send email. So the number of necessary relays will likely increase over time.