I don't think you can compare it to IPv6. IPv6 requires explicit adoption on the network side. In the case of HTTP/3, the vast majority of clients are already able to use it perfectly fine. They won't have any network firewalls in place that block UDP port 443. There are (virtually) no consumer ISPs that do that.

> If firewall changes are necessary for HTTP/3 to be adopted (eg connection tracking in routers may not work anymore), that can cause big problems in adoption.

If firewall changes are necessary for HTTP/3, then it's a slightly 'dumb' protocol in some ways.

If that's a prerequisite, then perhaps more effort should have been spent in getting SCTP and DCCP accepted and run HTTP over one/both of those. QUIC seems to have basically the same feature set AFAICT.

Besides the architecture decision of having QUIC being on top of UDP, are there specific features that are much better than SCTP and/or DCCP? Are the other two better in particular areas? How complementary or competitive are the two approaches? (Genuinely curious.)

The point you are trying to make is that people are irrational, therefore:

Even if the effort required to adopt something would bring in disproportionate returns, they would skip it because the effort is nonzero?

(just a generalization, no idea if H3 worth it or not)

The effort required may be less than the benefit as a whole, but as an individual probably not.

Moving from Python 2.7 to 3 involves a lot of work for some people with an old stable code base, and benefits those writing new code.

Of course as Python is open there’s nothing stoping you from maintaining 2.7 yourself - except that’s more work, and until now you’ve likely benefitted from the work others have done more than you’ve contributed.

I had some fairly new code written in nodejs recently that fubared when node was upgraded. Looking at the tangled web of code to try to get working again, and the fact it was under 13 months old before it was reduced to a valueless mess, I spent 4 hours replacing its function with a stable language as that was a better use of my time.

Firewall changes are only needed if your firewall rules were unreasonable to begin with.

> connection tracking in routers may not work anymore

What do you mean by this? Tracking of UDP flows has been supported by routers for decades.