> I think that garbage collection has proven to be the best idea in programming languages in the past decades, and probably the only one that has made an impact at all.

Depends on how you set your thresholds. Paying more attention to side-effects / purity / const has been pretty useful, too. And so have mainstream languages adopting first class functions. And making it easy to compose data structures. (Compare eg how Python allows you to return essentially two values via a tuple, and how cumbersome the same tasks is in Java; so people are tempted to use side-effects instead.)