good luck finding any major online property or infrastructure that isn't making use of some kind of proprietary syntactical dns sugar. it doesn't mean you can't span providers, but it does mean it takes a lot more work to do so.

anyway, you're not wrong, the best approach to mitigate this kind of thing is to leverage multiple dns networks. but doing so is not easy unless the application is still using dns like it was in 1995, and that is increasingly rarely the case.

Using a WWW subdomain with CNAMEs accomplishes effectively the same thing as using ALIAS on an apex domain name, and doesn't rely on anything out-of-spec or proprietary, making it easier to serve redundantly. (Did you ever wonder why google.com and facebook.com redirect to www?)

(Or is there more to ALIAS than that, which wasn't on the page in GP? Happy to be corrected if so)

you're correct about ALIAS (although practically, it doesn't matter: people are going to use the apex whether it's proper or not at this point). i'm more referring to other complex usually-proprietary capabilities of big dns providers, especially traffic routing features. routing semantics are generally not translatable across providers, and if you're using dns based routing (as most cdns, major web properties, etc are) then doing multi-network dns gets a lot harder. if you're amazon, you write and maintain a bunch of code to span providers. if you're not, the barrier to multi-network is high if you're doing more than static dns.

Yeah, that's a fair point. I'm not sure of a good fix for that, either.

You're right, but people want to get fancy with hosting at the apex (domain.com), even though it kills important functionality (CNAMES) forcing the adoption of hacks (ALIAS and ANAME records).