IPv6 has issues but it hasn’t failed.
One question I have about this statistic (and also about Google's IPv6 statistic) is, does traffic mean raw bytes? And in that case, is this just a reflection that like 75% of all internet traffic is just YouTube and Netflix due to video being a bandwidth hog?
(I'm a huge proponent of IPv6 - I just wish we had more useful statistics!)
I guess you're thinking that the number of v4-only websites is much higher than the traffic numbers represent -- which is true but that's actually fine because v6-only clients can still reach those sites easily via NAT64, so having a lot of v4-only websites isn't blocking deployment of v6 or undeployment of v4. The only real problem it causes is that people use it as an excuse to not do v6...
As for Google's stats, they're probably percentage of either connections or users as measured by their frontend load balancers.
I think the only way that we will move to IPv6 is a law (probably from the EU) that imposes to every provider to give its consumers an IPv6 address.