My MacPro 3,1 quite happily runs High Sierra using Dosdude's installers...
I'm pretty much with you. For someone to buy the OP's setup in 2019 they would have to know exactly why.
People that are still running 3.1's probably have overtime tweaked the systems to where they "work" for what is needed. You have to know how different SW & HW interact and work a-rounds. A prime example:
1. Exporting a 2:12 2.93GB clip out of Avid (8.10) H.264 takes 18:00
2. Saving the same file out of Avid as an AAF takes 00:03 secs. Importing the AAF into Resolve (15.3) takes 00:03 secs. Exporting to H.264 using CUDA cores takes 03:48!
Without knowing this "workaround", I could have cursed the 3,1 but the issue was Avid's playback/render engine. You get the same bad results across the board. There is supposed to be a "new" engine in Avid 2019.7
This was just 1 example on the video side. The only issue on the audio usually can be over come with RAID speeds and RAM. I use lots of NI instruments which are RAM hungry.
This is not to say there are not apps and things that just will not work on the 3,1. It's probably not on the minimum requirements list for a host of apps!
When you get into other fields, Houdini, Cinema 4D, MAYA etc. the 3,1 may be a big nogo!
But only 3,1 users get one of these from Apple each time you have to wait for the computer to process something!