1. ok, so it needs a bit of rework to multithread its process-starting system. I that significantly more difficult that rewriting the entire loader?
It needs more than just "a bit of rework", it requires a complete overhaul. Which is what SystemD is.
2. So it needs an extension to monitor services. Technically, I think this is better handled by a different task, one that is more into monitoring rather than blindly just continually-restarting a service that's crashed due to some external dependancy failure. Again, its not much of a task to add this than it is to rewrite the entire loader.
So, instead of one system handling bringing up a service, you want two. One to bring it up at runtime, and another to bring it back up if/when it crashes.
What SystemD is doing is, rather than using a stupid-simple for loop to start things, it's using an event-based system. This also means you can have things happen like "Network comes up, start network-dependent services", "USB printer plugged in, start print daemon", "Device hot-plugged, start dependent services". These are all things that are missing from the current SysV system, and require extra tools, along with their associated extra configuration and extra points of failure.