1. systemd (at least the PID 1 part) does not talk to the network, so a remotely-accessible backdoor would need to be more complex (and thus more likely to be detected) than a backdoor that can be loaded into a listening daemon like openssh.

2. You can run Debian systems without systemd as PID 1, but you're still stuck with libsystemd because so many daemons now link with it.