examples:
IPv6 migration. This is the best case, given that it was a necessity to combat the exhausting pool of IPv4 addresses. Even such only ~25% client adoption after 10 years. And worse support from websites in the alexa top 1,000,000 (https://www.internetsociety.org/resources/2018/state-of-ipv6...)
HTTP/2. Roughtly 35% adoption (by site) in the Alexa top 1,000,000. https://http2.netray.io/stats.html
HTTP/3. Obviously less.
Now I'm not trying to make this point from the position of the grumpy old man who doesn't want to learn something new. I love new tech, and obviously the research and development of new protocols over the past 30'ish years has created an explosion of new industry and communication patterns that have indisputably changed the world and shaped modern life.
However... the marginal benefit of each protocol advancement is directly related to it's usage (so I assume). The benefit the world saw (from the user perspective) when switching from HTTP/2.0 from HTTP/1.1 was arguably zero (I'm not saying it was exactly "zero", but I could argue that it was imperceptible). With bridge technologies like WebSockets and BOSH/Comet/"Hanging POSTS" it could be argued that HTTP/2 was effectively a bandwidth optimization in most users lives (those users that use the sites that offer support).
However, the additional complexity for developers to leverage these optimizations is detracting from the development of otherwise useful service offerings in the form of opportunity cost. This is because nearly every optimization inherently incurs a complexity cost, e.g. supporting both HTTP/1.1 and HTTP/2 is obviously more complex than simply supporting the former. And you are supporting it even if your deployment process abstracts the details away from you, e.g. debugging customer issues.
Formulating HTTP/3 when support for HTTP/2 is below 30% is simply an example of standards bodies working on a tiny "easy" problem (make the web go faster.. which is important) rather than face the bigger problem, i.e. helping to scale the worlds service providers a seamless migration path to adopt said new technology.
/rant from a grumpy old man