> (less of a problem today, with systemd everywhere)

Systemd is far from everywhere. Ubuntu 14.04 doesn't use it and is supported until 2019. Debian Wheezy is supported until 2018. RHEL 6 has support through to 2021.

Overall this means it'll start to be reasonable to presume systemd sometime around 2020, and be realistically almost everywhere from say 2025.

Software changes take 5-10 years to roll out everywhere. This is such a core change that it'll be on the longer side.