(Not saying I agree or disagree, just saying that this isn't so much an ancient artifact as it is a fairly recent and purposeful addition.)
-- NT is missing sendfile()? Also syscalls are fast on x86_64, most stuff is dynamic now a days and serving static files straight from page cache is fast enough.
• Kernel-mode request queuing. Requests cause less overhead in context switching
-- This one is somewhat valid but given you can ctx switch pretty fast on modern CPUs it is not clear how big the gains are.
Yeah but there would be some gains - that's not deniable. Question is whether they are big enough to risk something like the vulnerability at hand.
They were in use (at least TransmitFile was) by IIS long before http.sys was conceived.