The argument is that, if pid 1 dies, everything dies. Also a big pid 1 presents a big attack surface for nasty people.
Of course the exact same argument applies to a kernel: if something goes wrong in the kernel, everything dies and a big kernel presents a big attack surface to nasty people. However, I observe Linux is not a microkernel but it has a reputation for both reliability and being relatively secure. On the other hand, the quality of the people developing the kernel seems to be higher than those developing systemd, or at least that is the perception I get from reading all the hate on the Internet.