Which well-defined interface? Most Unix tools communicate through text pipes, and provide their result in whatever random format the author likes that then requires significant effort to parse correctly.
Traditional Unix tools have well-defined text interfaces. Well-defined =/= universal, or well-designed, or even easy to parse...some RFCs are rough to implement parsers/validators for, but I digress. My point is that it is easier to replace an individual Unix tool, that it is to replace a systemd subsystem.