In my opinion, macOS 10.8.5 running Pro Tools 10.x is stable and “rock solid.” Some of us put off inevitability to maintain productivity, but eventually “progress” (be it for our own benefit or not) will move us forward... or our older hardware will fail. For the time being, I’m gambling that a brand-new Mac might just as easily fail as a used one might. In a desperate last ditch effort to steal time from “progress,” I recently purchased a refurbished, downgradeable Mac Mini with a faster processor and more RAM than a new Mac Mini that would have cost me twice the price. If you think I’m being cheap, try to remember how much we generally get paid for our work, and understand that I’d prefer spending my money on mics, not Macs.
Downgrading to an earlier version of macOS isn’t a simple thing for the layperson, but the basic requirement is that you need to create a bootable installer from a macOS image file. If you’re familiar with the Teminal application, you can use the createinstallmedia command to generate a boot disk. Disk Creator <macdaddy.io/install-disk-creator> and the standard Apple-provided Disk Utility application can also be used. MacWorld <www.macworld.com> has published easy-to-use step-by-step tutorials, one of which turned me on to DiskMaker X. This application simply finds the original installer on your computer, then creates a bootable installer on most any external drive (USB, FireWire, or Thunderbolt), 8 GB or larger. DiskMaker X is available in Lion, Mountain Lion, Mavericks, Yosemite, El Capitan, and Sierra macOS versions. I created a handy Mountain Lion boot disk on a USB thumb drive to downgrade the aforementioned Mac Mini from Yosemite.
Whether you want to restore your Mac to factory settings or downgrade to an earlier macOS version like me, there are many advantages to creating a bootable installer drive. My rule is this: make sure you backup everything before you install anything. Things should go smoothly if you do a little research and read the instructions. You might also throw a few bucks to Guillaume for saving you some time.