Writing software that's safe even in the presence of bugs makes the challenge even more interesting.
The Postfix security model is based on keeping software simple and stupid.
Postfix keeps running even if one Postfix process dies; Windows requires that someone restarts the service.
At the time the Sendmail program had a very poor reputation with respect to security, with four root vulnerabilities per year for two successive years.
Qmail out of the box works fine, so people will want to use it regardless of licensing restrictions, even when the software does not ship with their system software.
One bug in an SMTP server can open up the whole machine for intrusion.
Adding functionality is not just a matter of adding code.
I don't expect an overnight change of all desktops to what the US Military used to call B3 level security. And even that would not stop users from shooting themselves into the foot.
In a previous life I wrote the software that controlled my physics experiments. That software had to deal with all kinds of possible failures in equipment. That is probably where I learned to rely on multiple safety nets inside and around my systems.
Most of the effort in the software business goes into the maintenance of code that already exists.