Alternative doesn't mean 100% substitute both ways. The pre-systemd didn't go in their abstraction in the way, that systemd does - it is basically system-level, event handling machine. The events could be not only boot, but also hotplug, timer, socket connection or others, and their handling is configured by the system or admin in unified way, including unified way of adjusting system-supplied units. Other inits just ran scripts at some order the boot and that's it, ignoring, that something doesn't have to happen at boot, but at some other time.
That's one of the things, that systemd handles way better, than any suggested alternative. By ignoring where systemd is strong, you won't make the alternatives any better.
> The only actual challenge is replacing just part of systemd while retailing compatibility with the rest of it.
No. I don't see why we should have the Sisyphos task of maintaing sysv/cron/etc idiosyncrasies forever. Sometimes, it is better to get rid of the collected warts, and not limit the system to existing semantics.
On the contrary, that's exactly what systemd's predecessor, Upstart, did. systemd, in contrast, did not. Basing the system around the concept of events triggering things turned out to cause problems, as discussed by Lennart Poettering and by Debian Technical Committee members during the Debian Hoo-Hah as their reason for not choosing Upstart.
See http://0pointer.de/blog/projects/systemd.html, section "On Upstart". It names two big issues with HOW.
This is wholly and totally imaginary. Every part of system has an alternative because it is all solving existing problems that people have been solving for decades with a variety of tools.
The only actual challenge is replacing just part of systemd while retailing compatibility with the rest of it.