> I feel like it won't be long before someone comes up with a crazy chimera of an OS that can run DOS, Windows, Linux, BSD, and macOS binaries.

This is the future, imho: some tiny, cross-platform, ultra-reliable kernel (like seL4), a set of shims that offer different sets of APIs (OS "personalities" if you will) to apps, an emulator, VM or binary translation layer here & there, all happily living under the same roof.

*BSD (including Linux compat layer) + Wine + some emulators like DOSBox already goes a long way. MS Windows desktop running on top of Linux isn't outside the realm of the possible either. And be sure to check Genode OS.

Fun times ahead!