systemd development is led by Red Hat, the $2 billion per annum gorilla in the corner. Third-party software, particularly from commercial vendors, typically us

systemd development is led by Red Hat, the $2 billion per annum gorilla in the corner. Third-party software, particularly from commercial vendors, typically uses Red Hat as the base system requirement. So you are correct, no-one holds a gun to distro maintainers' heads and forces systemd. They just find that there's no path forward without either [1] committing resources to keep abreast of the changes that systemd is creating both directly and indirectly or [2] just adopting systemd. Red Hat has won at this point through sheer persistence and clout, so most distros are going with option [2].