I was being somewhat tongue-in-cheek, but here is an example of what I meant:
Suppose your documents live in Wordstar for windows 3.1. So you keep Windows 3.1/MS-DOS 5 on a VM for a while. But one day you wake up to Windows 7, and it's 64-bit only, and won't your old DOS-supporting VM software anymore.
(I don't know if this is actually a problem. It's just an example.)
So you go ahead and install XP in a VM under Windows 7. On XP, you run a VM containing DOS, on which you run Wordstar.
Some years later, XP won't run on a VM since it's 2045 and nobody has heard of BIOS anymore. So you have a VM running Win7, which runs a VM running XP, which runs a VM running DOS, which finally runs Wordstar.
Then 25 years later, your VM software doesn't work, so you add another layer...