[mr_luc]

    This is a hard market to time. Nobody 
    believes the Internet will be IPv6-only 
    within the next few years. There are 
    credible people who believe IPv4 addresses
    will be scarce and useful indefinitely. 
    We might put money on you getting away 
    with an IPv6-only app 20 years from now.
    But we'd have said that 20 years ago, too.

This was interesting to me, so I looked up adoption metrics.

In 5 years (2016-2021) it went from 9% to 35%.

Source: https://www.google.com/intl/en/ipv6/statistics.html

TL;DR: it's being adopted at 5% of internet traffic per year, over the past 5+ years.

Once it gets to 80% or so, some number, there plausibly could be a flood of "we don't support IE6" behavior where adoption accelerate.

But that would still be 9 years, if the rate of adoption didn't accelerate at all.