I observed the same thing when I recently considered selling mine. When I realized what I'd have to let it go for, I changed my mind. Glad I did too. Just glad I'm not in a situation where I need to sell it.
When I bought mine in 2013, there wasn't much in the way of competition other than the Versys 650 which was kind of strange looking. Since then, the Versys 650 got some nice updates, along with the addition of Versys 1000, NC700X, CBR500X, AT and FJ09. A lot more competition in the arena.
It's not just a VStrom thing. It seems all bikes are suffering the same fate.
If I had the money, I'd be scarfing up on some good used bikes right now.
Bike sales have dropped nearly in half since the peak in 2006 of around 700K per year to 370K in 2016. Unreal