You write replacements for or properly port your every-day workflow to the new OS. You already wrote a whole new OS for some reason even though there are hundreds to choose from, presumably there is value in replacing your tools to take advantage of whatever you put all that effort into or else why bother? The VM is for things you haven't ported yet or less important workflows.

Besides, people run windows and do all their work in WSL all the time.

> Writing any useful application program in a complete void, without standard libraries and utilities, would take a very long time [...]

So does writing an OS and you've already decided that was worth the effort, yet you balk at rewriting some commandline utilities[0] and a standard library? Please.

> It is much easier to first write a new foundation [...] then start to use the foundation with the existing programs [...] and then replace one by one the programs that happen to be important for you and that can benefit the most from whatever is different in the new OS.

Any reason not to just do that with a VM? Forcing POSIX compatibility into your OS is going to constrain your choices (not to mention your thinking) to the point that you'd probably be better off just modifying an existing OS anyway.

> For the vast majority of programs that you might need from time to time it is likely that it would never be worthwhile to rewrite them to use the native interfaces of the new OS, but nonetheless you will be able to use them directly, without having to use complicated ways to share the file systems, the clipboard, the displays and whatever else is needed with the programs run in a virtual machine.

A: it isn't that complicated. B: if you can use them so directly without having to deal with the separation provided by a VM, it's likely you didn't improve their security situation anyway. Again, why not just modify an existing POSIX OS in this case?

> Implementing some good methods for seamless sharing of data between 2 virtual machines, to be able to use together some programs for the new OS with some programs run e.g. in a Linux VM, is significantly more difficult than implementing a POSIX translation layer enabling the C standard library and other similar libraries to work on the new OS in the same way as on a POSIX system.

I doubt it is as hard as, say, writing a brand new OS that's actually in some way useful. Why go through the effort of the latter only to throw away a bunch of potential by shackling yourself with a set of barely-followed standards from the 1970s?

[0] POSIX does nothing to help you with anything GUI.