Don't you think they should continue to upgrade Windows 10 - particularly as many machines won't support Windows 11?

Most likely new Windows releases are now a driver towards people getting newer hardware. I think very few if any new features will be backported.

But I'm sure they'll keep up the half-yearly updates to Windows 10 until it is retired in late 2025.

Yep, for security related fixes.

Windows 11 running perfectly on three old supposedly incompatible machines here. I am now getting messages that upon releasee I will "Be advised" to do a clean install of Windows 10, so many machines will be happily running Windows 11, where secure boot et al which are all bypassible is more about software control than machine capability