And remember... it may not be worth much to some but it will ALWAYS be worth more than a second hand soft synth - depressing but very true. Furthermore, it will still be working 20 years later, something that can't be said for your average PC and support for it's Operating System!!
A sentiment repeated often and I'm not sure what it's based on.
Right now, people have ROM players for old, old, old school video games and just about every OS has a legacy mode. If not, the users themselves will build a host. If there is ANY demand for a VST player that supports "outdated" software, someone will build it.
And worse, you can still install old operating systems on moden systems (using dual boot if you have to). The idea that I may or may not buy a VST for a concern about 20 years in the future is a little far fetched.
Aside from that, I hope I'm using different gear 20 years in the future.
I can see keeping a piece of two, but really...
There are artists who have made their hit records with a particular set up and they still modernize or change up their studios. I saw a Wavestation and DX7 for cheap and still didn't jump. I think it's a combination of all the reasons mentioned. Synths that are workhorses and main instrument/sound suppliers are hard to let go.
The sprinkles on top units are easier to replace.