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One gets the impression that optionality is only theoretical. Same for separate binaries. The degree of coupling in the systemd architecture is enormous. So then, what purpose do separate binaries serve other than being able to (conveniently) be used for deflection or provide a certain facade?

The philosophy of systemd exemplified through PRAXIS is one of subsumption and uniformity (== taking away choice) under a singular vision defined by the systemd implementors. In practice, this means that you are penalized in various ways if you don't "buy in all the way". You can take a look at all the distributions that ship systemd by default and see how many have bought in all the way vs not using the "additional functionality".