To argue the other side, if you don't like something in OS X or Windows, you can type "vim my_os.cpp" and start implementing your own version from scratch.

If three years from now, everything from Firefox to GNU tar depends on systemd, then your open-source Linux is in the same boat.

[Edit: Just to be clear, I don't expect that to happen. I'm just pointing out why people get upset when systemd encroaches on very large numbers of system components.]