It doesn't really surprise me if people are having more issues on the newer OSes, if the software was originally written for the older ones.
I imagine that may be part of Access' motivation - it is more important for them to ensure their software works properly on the platforms people are migrating to than to maintain functionality on older systems. Ideally, they would be able to manage both, of course, but I have no idea what their code, requirements or the libraries they use look like.
I will admit that I don't exactly produce much, I'm a hobbyist that always loved the Virus sound since I heard it in the 90s and bought one when I could afford to a few years ago (on that note, believe me when I say I can appreciate your feelings about the price of the Virus!).
My reply was made more with my own perspective as a computer programmer as opposed to a synth programmer or musician (sometimes I get the impression people don't appreciate why a developer might make a decision like Access may end up doing, so I felt compelled to point out some of the probable reasons).
That being the case, I'm sure I don't understand a lot of the issues you and others may have with migrating to a new system while in the middle of a project. From my point of view,
it technically shouldn't be a major issue, so long as all your software works. I'm sure there's far more concerns than that though. So, I certainly don't mean to criticize anyone for lobbying for Access to not take a path that may cause them issues in their work.
Regarding the Flash ROM, I seem to recall having asked about this some time ago, and that I was assured that there wasn't much reason to worry about it running out of "writes", so to speak.
Regarding the polyphony, I'd expect the polyphony, if anything, would improve for patches not using OS5 features, but it's certainly possible that it would degrade; for instance, if a bug were introduced, or even if a bug were fixed, if the fix required changes that had a performance penalty. So without testing, I have no real answer for you on that.
So far as installing multiple versions of the OS on one computer, which is what I assume you meant by VST2 and VST3, and whether that would allow you to select between them in your DAW, I'm not certain.
It seems to me that the filename of the dll that is installed for the Virus VST does not seem to vary based on version, so you'd at least have to have to do something to keep them from overwriting one another on installation, either move them to different locations or try renaming them.
I don't think there's likely to be a reason the path or name of the .dll should matter to your host, since I imagine your DAW doesn't assume a particular path/name, but rather just gets paths from you, and grabs .dlls from those locations. Therefore, it shouldn't care about names or any such thing. Assuming your DAW has you select VSTs via the .dll file names, you could then pick between them if they had different names. Just to be clear, I'm referring to the VST .dlls, e.g. Virus TI.dll or Virus TI Snow.dll, not the Virus Control.dll or Virus USB.dll. I suspect the VST .dlls probably use Virus Control .dll and would not work if that were moved.
Note though, I highly suspect that even if you had all these .dlls installed on your computer, you'd only be able to *run* with whichever matched the OS the Virus itself was running; it just seems likely there would be some changes in the communications between the VST/dll and the Virus. Actually, the Virus Control.dll and Virus USB.dll could very well change as well, thus even if the OS on the Virus didn't matter, those files might have to have multiple versions and you'd possibly have to swap them back and forth depending which version you wanted to use.
The above is all guesswork on my part though. It would be much better to get answers on these questions from Marc/Access. I'm actually curious about this myself, now that I've gotten thinking about it.