If they plan to keep selling their products in the future they should update their plugins. Already today they are forced to update every time Apple come with a new OS or if there are other major OS updates. 4K is getting more and more common and soon enough you can't buy a new monitor unless it's 4K or more.You can't expect every developer out there to update their plugins which are over 10 years old just to support the newest tech.
Your idea to use a separate monitor just to display plugin GUI's is not a bad idea. I still have my old monitor, but I need to find space for it somehow. I tried before to somehow change settings with this new monitor and change resolution but it became a mess.
I know this because I recently bought my new monitor and very few monitors were not with 4K resolution. Here in Hong Kong we have a place called Golden Computer Arcade which is a huge mall with many small computer shops in it. I been looking around.
Just for the heck of it I added a photo to show Largo and I put Valhalla Vintage verb next to it for size comparison. Valhalla is possible to re-size but I just loaded it up with default size. As you can see the GUI is a bit small but the really difficult part is the text.
