From this point of view I don't think the opposite of "simple" is "complicated" but rather "full-featured". Softwares who try to follow the KISS philosophy ofte

From this point of view I don't think the opposite of "simple" is "complicated" but rather "full-featured". Softwares who try to follow the KISS philosophy often implement the usual 80% of functionalities that take 20% of the time, and leave the rest to other projects.