Equally or more pertinent: What does an office admin, an assistant merchandiser, a product designer, or a copy editor use their desktop for?

They use a Web browser, email, calendar, word processor, spreadsheet, an image editor, and a dozen smaller proprietary apps that are absolutely essential to their job.

I think it may be the last of those... all the various industry specific apps that are hindering Linux as much as the obvious big players like Microsoft. I think word processing on Linux is fine; but your company's eight year old customized CRM tool will never, ever, be available on anything but Windows. Or specialized document scanner. Or sales commission tracker.

In my experience they don't use "an image editor" or "a spreadsheet program", but there is just exactly one blessed app. If that app is on mac for example, then windows or any other OS is impossible.

When people say that "X has no alternative for Excel", that X includes Windows as well. As a good rule of thumb: If there actually is an alternative on Windows, then there is also one on linux.

Anecdotally, I've done work with a few medium-sized businesses around updating legacy business critical apps now and in at least one case the app just wouldn't run on Windows 10 at all but worked fine in Wine with no special configuration.

Microsoft appears to have taken a step back from their long-held focus of not breaking things on upgrades.

Granted, these clients should not be using Win9x apps still and hoping for compatibility.

Well, good for you (work/pay) and good for them (security/efficiency), win win.

I do feel like this becomes less and less of an issue with many of these bespoke apps becoming web apps. Certainly there are still a lot of old native apps that you might not be able to run on Linux, but there are far fewer of these than there were even 5 or 10 years ago, and especially 15 or 20 years ago when the web browser didn't offer enough functionality to build complex, performant apps.