Your last point first: Am I arguing that "we shouldn't use such [explicitly never agile] protocols"? I don't see that.
I'm arguing that the case for them is weaker than is often put, but that's not the same as nobody should use them. If a flag day is fine for your use case there's very little reason not to choose this design approach, it is simpler and simpler is good. But you'll notice that the example cited (including by you) for why agility is bad is almost invariably TLS and clearly a flag day isn't practical for TLS because it's far too broadly used.
TLS illustrates my other main thrust of concern on "agility is bad". You describe RC4 as "known bad" and the downgrade attacks as "catastrophic" and this sort of apocalyptic thinking is very popular in the "agility is bad" crowd, but it doesn't truly reflect the ground reality for actual users which is that things went from "It's definitely fine" to "It's probably fine but to be sure we should upgrade". Grey areas are a real thing.
There were protocols that didn't exhibit any cipher agility before by the way. Lots of them. What happened was that they broke, and so agility was added to them retrospectively in new versions that fixed the brokenness. The arguably new thing in the latest round of "no agility" protocols is a supposed determination never to do this. To see how that works out, as I said, you'll have to wait a decade or two.
I'm arguing that the case for them is weaker than is often put, but that's not the same as nobody should use them. If a flag day is fine for your use case there's very little reason not to choose this design approach, it is simpler and simpler is good. But you'll notice that the example cited (including by you) for why agility is bad is almost invariably TLS and clearly a flag day isn't practical for TLS because it's far too broadly used.
TLS illustrates my other main thrust of concern on "agility is bad". You describe RC4 as "known bad" and the downgrade attacks as "catastrophic" and this sort of apocalyptic thinking is very popular in the "agility is bad" crowd, but it doesn't truly reflect the ground reality for actual users which is that things went from "It's definitely fine" to "It's probably fine but to be sure we should upgrade". Grey areas are a real thing.
There were protocols that didn't exhibit any cipher agility before by the way. Lots of them. What happened was that they broke, and so agility was added to them retrospectively in new versions that fixed the brokenness. The arguably new thing in the latest round of "no agility" protocols is a supposed determination never to do this. To see how that works out, as I said, you'll have to wait a decade or two.