Back in the old days, insmod and modprobe were synchronous. If you wanted concurrency during boot, you could use fork/join to get it (using the & line suffix and “join” command in bash, for instance).
In the brave new world, all kernel events are published over some wonky bus thing, so you need to write a daemon that listens for hardware events and replicates the kernel’s current state of the world in dram, and also is fault tolerant, and has a security subsystem that authenticates user requests before forwarding them to the kernel.
The new approach is fundamentally broken because it is too complicated to understand or implement correctly.
Also, it is gratuitously reimplmenting basic kernel functionality (security, process lifecycle) in user space, and in at least one case that has hit me personally (DriSetMaster) the systemd contingent has been blocking straightforward patches to the Linux kernel that make it possible for non systemd init systems to work without adding their own crappy shim processes.
I run Linux because I want a working unix environment. Systemd is turning it into something else.
I think it is telling that we’re seeing things like fuschia and renewed interest in the BSD’s specifically because Linux on the desktop / laptop is increasingly an unsupportable mess.