My 2009 MacPro won’t die. It is old fashioned thinking to want to keep a machine til it breaks. Maybe need to rethink this. Upgrading it to Sierra will strain its creaky joints so maybe it’s time.
When they get slow, they are dying. I’ve noticed this Mac runs a tad hot. Over time, heat degrades the components. An 8 year-old machine is admirable but they tend to die abruptly.
This Mac was free so I try not to complain. But it is too tempting to turn it into an expensive Windows machine.
My 2008 (3,1) Mac Pro runs Sierra just fine! Although I did add an SSD and 16 gigs of RAM. "GeekBench" of 12,500 or so, similar to some 2017 Win 10 boxes with an i7 (Intel Core i7-4960HQ). Best $800.00 I ever spent! (Got it "used" 2 years ago from a navy sailor/Hornet mechanic, got a discount because it had a dent in the upper aluminum handle).