These points have been made before. This does not alter their validity.
Arguing that reliability in a critical system component is not of concern is absurd. Yes, the linux kernel can contain bugs. If you do care about reliability, you'll take steps to mitigate the chance of failure. This is a completely separate issue to the robustness and reliability of PID1; there's no need to confuse the discussion by mixing them together.
Ignoring the fact that this is an important concern does not do systemd any favours. systemd could certainly be changed to move the vast majority of the complexity in PID1 to a separate program running in a separate process. There's no intrinsic need for anything to be in PID1 except process reaping, starting/restarting another process and handling shutdown. Everything else can be done in another process; even shutdown--you can just exec the shutdown program. Take a look at how s6 is structured--it has a lot to be said for it, and there's no reason why systemd can't do this.