I’m reading reports in some forums I trust of 2009 machines with upgraded 2010 firmware are installing High Sierra without issue. Seems you have to have a MAC video card to do the install but after the initial install a 3rd party nVida card will work just fine.
Since I never install a new OS for at least a year after it’s release I’ll be fine. When Apple does cut me off from OS upgrades I will just use Windows 10 in place of macOS. The machine works just fine for my needs in pro audio either way. I just prefer OS X or macOS.
People always tell us how stupid we are to buy Apple. It isn't the hardware it is the software. When you can use an 8 year old machine to run the latest software how expensive is it really?
If you aren't running an SSD startup disk you should, makes the Old Mac Pro run like a new machine. I am using three in mine, one for a startup did and a couple for record and compress disks, makes a huge difference.
The new Mac Minis are dog slow and pretty hard to upgrade. You have to disassemble the entire machine to change out the drive, but you can run from an external SSD through the USB 3 port and make it much much snappier.
I use a PCI card I got from Other World Computing that has two SSD drive slots and 2 external eSATA drives ports. USB cards can be problematic some work until the next upgrade then......