Under what circumstances does the choice of backend OS ever deliver "explicit customer value", so much so that the customer would care about said choice?

Normally the customer doesn't care. They will care, however, if something goes wrong during the migration or some unforeseen issue comes up later that degrades their experience.

It is definitely the case that underlying architecture, certainly all the way down to the OS, can be the difference between "I can create, test, and deploy this feature in a week, and it will be rock-solid" and "it'll take months and still fail on some edge cases—and fixing those is not remotely in our budget".

I'd hazard to say that it delivers customer value via two avenues: (1) better SLAs, and (2) lower expenses.

If your backend OS starts to run software more efficiently, cost less to host, has less maintenance downtime, has fewer security incidents, has fewer crashes, etc, having changed to it has produced customer value.

> Under what circumstances does the choice of backend OS ever deliver "explicit customer value", so much so that the customer would care about said choice?

Is the service working, is it not? (are there show stopping OS issues?)

Are the performance characteristics we need there, or not?

Is the service secure or not?

There's a whole lot of things right through that entire space where the choice of operating system can make quite a fundamental difference.

As a general statement, I wouldn't want to make this kind of a migration without something bordering on a killer feature that wasn't possible otherwise, or some fundamental driver problems (I've replaced Linux with FreeBSD on border devices in the past, due to particular issues with Linux in ways I couldn't afford to have keep biting me)

is this a joke..... this must be a joke

Quick: what OS is running on the machine at your bank that makes your debit card work in an atm? If you have to care about the answer to that question, your bank is doing it wrong.