What he accomplished was to teach otherwise isolated civilizations that unless they put aside their differences and worked together they were vulnerable to devastating attack.

One point I liked about Alternate History Hub's version of this is that they pointed out that very few people actually knew what happened. Imagine if half the people on Earth today turned to dust in an instant, and imagine this was a world where Marvel had never existed. Most people would interpret that as some sort of act of Divine judgment. A quote I loved from that video was something like, "people will come to many conclusions. Giant purple man with magic space gems is not the explanation they'll go to first...or ever."

I don't think Thanos taught anyone anything. Billions of species throughout the universe saw half their population turn to dust. Then a few years later, all those people just mysteriously came back. They have no idea what a Thanos is. They've never never heard of the Avengers or the Infinity Stones. It's just this completely inexplicable event outside all known laws of science. It's unambiguous proof that some sort of supernatural forces exist in the universe, but everything beyond that is open to interpretation.