Splitting something into multiple executables doesn't make something not monolithic just like splitting a program into multiple source files doesn't make it not a monolith. It's not a monolith when the different component parts can be cleanly factored out, replaced, and used outside of the original context or with different versions of related components This is in fact very hard. Much harder than making a monolith.
For instance the X11 ecosystem isn't a good example of a monolith because its designed to make it trivial to swap in different loosely coupled components. You don't worry about needing your window manager and X11 to come from the same commit so they actually work. You can argue that the toolbox that systemd provides it is worth it but arguing that it's not a monolith just screams I'm not a developer.
People have been making services for decades and systemd is 13 years old. I kind of feel like it probably has virtually all of the options its ever going to have. Also most of what people would use such a GUI for is to start stop restart enable disable the thing people have been doing for an eternity which doesn't require even displaying the unit file.