I think this is a great topic.
I imagine from the email that they want to install Foo OS and the latest and greatest apps, but they want their desktop and apps to behave like they did under a previous OS.
If they had a great workflow with a previous product and OS then good for them, why break it? Sometimes modern doesn't always mean better. A stupid phone might actually be easier to make calls on.
I see this again and again. Foobar2000 being an example of an app that had many useful features for me. I found it difficult to replace under Linux, even apps that came out years after didn't satisfy my needs. I was even thinking about using it today for adding replay gain to a folder of music. And as such I have it installed under wine.
The other classic example is users forced to transition form WordPerfect to Word. My Dad reckoned he could do everything in WordPerfect (and told me time and time again), and loathed Word with a passion. He'd mastered his tool. And once you have done that, you are reluctant to change. And in that case there wasn't that much of advantage for him in using something else (there may well have been for his sysadmins).
In an ideal world apps would be built with a software interface that you could programmically explore to build your own UIs around, so you could build a UI in a certain style. Then under your distro you could flip a behaviour switch - to emulate an Amstrad desktop say, and Firefox there would just blend in. My gut says that the toolkits used to build apps are too tightly coupled with the code to do so, but that's just a guess.
To get the feel of an OS of old, you'd want to replace your file manager and launchers too. But why not!
I don't know about monetising the idea. There might be a solution for them in using an emulator. Sadly though there are some constraints with older programs, perhaps they don't support newer protocols or lack security patches.