Please elaborate what programming language you think isn't a fad and isn't a playpen and why. Restrictions on languages IMHO clearly narrow what you have to think about other code doing. Whether it's marking a field "const" in C++ or having the borrow checker in Rust or having private fields in Java or immutability in Clojure, all those things make it easier to know what a large system of code /cannot/ do, and that makes it easier to maintain. That has nothing to do with people other than it might be you fighting years of code that you wrote yourself.