> the reality is systemd is a bunch of individual modules
I disagree. Technically you are correct, but the same modularity argument can be made for practically any piece of code bigger than "Hello World". However in practice systemd is shipped as a monolith. I just checked, and even on Genoo with its uber-flexible USE flags and compilation from source, you can't shut off individual features like logging, dhcp, ntp, etc. Most people just install the binaries.
No, systemd is not the end of the world. But it would be the end of running my machines the way I wish to - at least without spending more time and effort keeping it fenced in as you suggest.
Systemd has a configure system similar to the kernel. Like the kernel, where some distros use a "full" kernel and others use a stripped down version, it is up to the distro to decide what fits its need. The same is true with systemd. Take distros running late versions of gnome-shell, which requires the logind module of systemd, all they are including is logind, not all of systemd. Even Ubuntu is using logind with upstart and also sys init scripts.
Nobody is forcing a distro to adopt systemd. If the distro developers do and it impacts you, the complaint should be with the distro developers, not systemd. Maybe the developers have good reason for making the change. Most of them are community based, so it is doubtful that they are doing it just because Redhat is doing it (although Centos and Scientific would most likely change to maintain compatibility).
Even if your distro of choice does change, it shouldn't force you to stop running your machines the way you want too. You can freely mix systemd, upstart and init scripts, using whatever pieces you want for whatever need you have. That applies whether your system is a desktop, server, embedded or whatever.