Furthermore, the lowest adoption tends to be in developing countries (Africa, parts of Asia and South-America), and I bet simple financial reasons are a decent part of the reason for that, if not the primary reason. The Indian government has invested quite a lot in IPv6, so it's leading in adoption rates last time I checked. This is good long-term thinking IMO, since developing countries are also the worst affected due to less addresses being assigned to them, but not every (developing) country has the means for that, and there are often other, more pressing, matters to deal with too.

Carrier-grade NAT (multiple consumers sharing the same IPv4 address) mostly averted catastrophic problems though. It's not perfect, but it works.