Before 2005 experts estimated that AIs that could beat humans at Go would take many years still. In October AlphaGo came out and in March 2016 beat everyone. In 2017 AlphaGo Zero came out, which was only trained on self-play. It beats all previous versions.

But it is not so much these big breakthroughs that cause current optimism, as much as the large amounts of continuous improvements and refinements.