Systemd doesn't just replace 1000 lines of Sysvinit. Those "millions of lines" also replace the thousands of hundred-line poorly-vetted Bash scripts you need, plus your syslog daemon, plus dozens of "fork, setuid, chroot" variants, plus your cron daemon. And that's before we get into things not directly related to service management, like logind, hostnamed, networkd, resolved, timesyncd, tmpfiles, udevd, and so on.

Systemd is an umbrella project. Look at each individual component in isolation, and the vast majority of the time the thing it replaces is significantly more complex and fragile. Systemd was adopted by most of the Linux ecosystem because it was a significant improvement over the status quo, there is no need to invent conspiracy theories here.