I suspect that at some point, there will be a necessary move from VM-as-supervisor to full emulation.
For example, if you're using VirtualBox, then at some point the version of Windows may no longer be supported as a client OS. That's the time to shift the image to something like QEMU.
I should point out I wasn't envisaging the idea of a VM disk image as long-term storage, more as a transport medium. If there's genuinely no other way to get the data into the VM to be used, then simply giving it a fake hard disk is the ideal method - use a more modern VM to save the data to the disk, shut that down and then present it to the VM that has the software you need.
I envisage the data itself being seperate to the VMs themselves in all of this - the VMs should be small "access points". The disk image idea is just a way to get data into them temporarily.
So, to be clear, we have a two parts to the solution - your storage, which you can do what you want with. Keep multiple copies, keep checking the medium is good (via md5sum or similar), and so forth. And the access system, which is a VM you check once a year. And if it needs to be updated/transitioned to emulation, at least you know and can deal with tha.
On a very large scale, this divides the work between two teams - a storage team maintain the actual archives, and an apps team who maintain the access.
Of course, this is only if we want full fidelity. If we're OK with bad reformatting by a later version of the program, then we don't need the second team at all. :-)