They should also use just one domain - DNS lookups are a pretty slow and flaky part of the process of loading a webpage, and downloading loads of HTTPS certificates for all the domains involved isn't great either.
Managing just one domain is also a lot easier - less to monitor, less to maintain, simpler architecture, etc.
The only remaining reason for extra domains is to act as a security boundary - for example when you're running untrusted user javascript.
> Nobody has an excuse not to use either HTTP/2 or QUIC on a modern website.
Sure they do. HTTP/2 is not uniformly superior to HTTP/1.1, primarily due to head-of-line blocking; so that on a poor quality network connection, with multiple HTTP/1.1 connections it’s quite possible that one will come through while another takes longer due to packet loss, while on HTTP/2 it’s all on one TCP connection, so that packet loss affects all requests. That’s what QUIC is about fixing. HTTP/2 is also currently incompatible with WebSockets (there’s a draft in progress for resolving that), so HTTP/2 offers no benefit for WebSocket domains or little-to-no benefit for WebSocket-heavy stuff. HTTP/2 also works differently from HTTP/1.1, so that certain moderately obscure combinations of types of long-lived requests can’t be cleanly terminated in HTTP/2 as they can be in HTTP/1.1 (sorry I’m being vague—I can’t recall the details off-hand). And there’s just the usual “existing software that doesn’t support HTTP/2” justification as well.
(Despite the listed criticisms of HTTP/2, I do agree that almost everyone should be using HTTP/2.)
As for QUIC (Google or IETF variant), it’s still experimental, with various unknowns that are being nutted out, and is not suitable for wide deployment by most entities just yet.
> The only remaining reason for extra domains is to act as a security boundary
No, that’s merely one of the best justifications for additional domains. There are other reasons for using other domains, though I would agree with a less strongly worded version of your initial declaration: that they should try, if convenient, to use just one domain.