Go is #12 on the TIOBE index, and by that measure more mainstream than Perl or Ruby. If by mainstream you mean "the top 3-5 programming languages", then you're not talking about anything invented in the last 20 years, which is kinda the point.

New languages can reflect mainstream opinions even if the languages are not mainstream. Mainstream languages tend to represent fossilized opinions from the time when they were designed.

If we were to use a car analogy: go has been in production for the last few years and there's a ton on the road, but the average car on the road wasn't built in the last few years.

The mainstream is not only defined by what's most popular, but also by what sets the trends. The masses always get the message with some delay...

Besides, all of the three languages that you've mentioned are turning more functional and less OO with each year...

JS still has OO (and recently got "classes" as sugar), but it's not used in the crazy deep-OO-hierarchy style at all.

Java, that used to cherish that style also abandons it nowadays (with closures, streams, etc.).