I hear this argument a lot, and fail to be convinced. Optimizations at scale matter a lot. Let's posit that 100,000 developer-years have been spent implementing HTTP/2 and tools to interact with it. (Hopefully a great over-estimate). Let's say we have to spend all that again to move to HTTP/3. Is it worth it?
Well, per random internet stats I haven't validated, there are about 4b internet users, spending about 3h per day on the internet apiece. I could imagine 5% of that is spent on synchronous resource loading. That's about 10m/day/user - or about 28M years/year spent waiting on resources to load. A 1% savings (the minimum of CloudFlare's range) is 280k human years / year. And that's the bottom end - could be as high as 4% (1.1M years/ year), and without BBR (negatively impacting high packet loss and high throughput connections in particular).
Is "persons average time on the internet" comparable to "paid developer time"? No. A lot of that user time is timeboxed "wasted" time, in the endless content scroll. With that in mind, is it worth it overall? Probably. I've tried to estimate in favor of developers, but I still get their one-time effort paid back 2.8-11x _each year_ in user time, under unfavorable conditions. And this is growing rapidly, both in user count and time per user.
And that says nothing about the privacy benefits of encrypting headers and whatnot, which you should probably ascribe nonzero value.