I envision KISS as a minimal base in which you extend to suit your needs and not something you need to cut to size.
As I state in the Philosophy; "it's easier to add things to a system than it is to remove them".
A user has actually gone ahead and done this! \[1\] They run KISS with systemd, pam, dbus, pulseaudio, glibc, chrome etc etc.
So the project is an arbitrary reduction of scope and then writing the implementation just for that scope.
Imho, the issue is not that the solutions are complex but the problems we (assume we) need to solve are. Just making simple solutions that ignore the bigger problems underneath do not really help in the end.
That doesn't make the original system any simpler or more complex.