What use are the tests if the software you're testing is obsolete?
The 'we have a lot of tests' argument would have been good if systemd were a new project without any users. But right now systemd is used in pretty much all other major distributions (that had time for at least one release cycle after the systemd introduction).
Let's see: OpenSUSE uses it, Fedors uses it, Gentoo supports it, Debian supports it, RHEL will probably support it in the next release.
So the decision to use Upstart is purely political, not technical.