Say you are working on a banking system. You ship a login form, it is deployed, used by tons of people. Six months later you are mid-sprint on the final leg of a project that will hook your bank into the new FedNow system. There are dozens of departments working together to coordinate deploying this new setup as large amounts of money will be moved through it. You are elbows deep in the context of your part of this and the system cannot go live without it. Twice a day you are getting QA feedback and need to make prompt updates to your code so the work doesn’t stall.

This is when the report comes in that your login form update from six months ago does not work on mobile Opera if you disable JavaScript. The fix isn’t obvious and will require research, potentially many hours or even days of testing and since it is a login form you will need the QA team to test it after you find another developer on your team to do a code review for you.

What exactly would you do in this case? Pull resources from a major project that has the full attention of the C suite to accommodate some tin foil Luddite a few weeks sooner or classify this as lower priority?

This is a great example... except I think the right answer to "what exactly would you do in this case?" doesn't support your argument.

I'd document that mobile Opera with Javascript disabled is an unsupported config, and ask a team to make a help center doc asking mobile Opera users to enable JS.