I'm pretty sure IPv6 adoption will follow the standard S-shaped adoption curve [1]. If true, that sucks, because it means that the last 10% will take approximately as long as the first 10%.

Intuitively, I feel like that could in fact be the case. The last 10% to switch will probably be small ISP's without resources/knowledge, old devices that don't support IPv4 and old servers/webapps that haven't been updated in years.

[1] https://en.wikipedia.org/wiki/Diffusion_of_innovations

But won't it reach a point where adoption is so high that major backbones and carriers simply retire their IPv4 equipment?

At that point you adopt or die.

There are already carriers that are looking at taking IPv4 in at the edges, encapsulating it in IPv6 and then decapping it before handing it off to their customer.

Thereby the core network can be entirely IPv6 without any legacy IPv4.

Eventually, yeah. E.g. a lot of ISPs have shut down their Usenet servers over the last few years.

But it could easily take another 20 years to reach that point.

It rockets to the high percentiles but never quite 100%, that's where the real annoyance lies.

Well off the bat you can already rule out the idea that you'll have 100% IPv6 adoption in the next 20 - 30 years. There are simply too many embedded network appliances that are IPv4.