It's been quite mainstream in certain contexts and geographies for a number of years now. As an example - most handsets on modern LTE (and newer) networks have been strong-majority v6 for quite a while. The fact that this hasn't been obvious is an argument in favor of v6's success.

The expectation is that networks will switch to IPv6 only internally, and eventually the IPv4-only remainder of the Internet decays until it's no longer an "IPv4 Internet" but just a handful of separate IPv4 networks that are connected to the (now IPv6 only) Internet by protocol converters.

Some US corporations did this already, rather than fuss with being "dual stack" and potentially introducing new IPv4-only services or systems, they switched wholesale to IPv6 and add converters at the edges. By choosing to do this they get most of the benefits of a future IPv6-only Internet today. For example, numbering internally is a breeze, they can auto-number almost everything because the address space is so vast there is no need to "plan" any of it.

Lots of other US corporations are still IPv4-only, indeed that's why the Google graph earlier has a distinct weekday vs weekends / holidays step change in it. At home a very large proportion of people in industrialised countries have IPv6, major ISPs supply it, common household routers understand how to use it, every modern OS groks it. But at work IPv6 is often disabled by policy, in favour of cumbersome IPv4 because that works and changing things at work is forbidden.