Doing it yourself is fine as an educational exercise for newbies, but skilled linux users generally have better things to do than to do the setup by hand for the nth time. On the other hand the “vanilla”/bleeding-edge approach of Arch makes it one of the best bases for derivative distros available, so basing your distro on it is a no-brainer for many.

“Manjaro is not stable because it ensures no breaking updates are pushed to users” is such a weird statement to make.

does not comply with the principle of K.I.S.S. One application should solve one task and can be replaced

That’s not KISS, but the UNIX principle. And even that part is wrong, as in traditional UNIXes applications were certainly not replaceable.

The features themselves are very useful for basically any user. Whether they are worth the non-standardness and issues that come with it is another question.