As with all technology, things began to fail on my 2009 Mac Pro tower, starting with the dedicated PCIe SSDs for streaming virtual instruments. I also couldn't run a current operating system, and there were so many bugs that I finally bit the bullet and migrated to a new machine.
I'm a big fan of buying slightly older equipment, as the depreciation is so crazy after a year or so. After way too much consideration, I went with a 2017 iMac Quad-Core 4.2 GHz with 64GB Ram (maxed out). It has a 500GB SSD boot drive dedicated to OS and Apps. My 3TB Raid Data drive is connected via Thunderbolt 3 and housed in a OWC enclosure along with the 2TB Backup Time Machine. I took those drive directly out of the old Mac Pro and plugged them into the enclosure... and it all worked. Just a little bit terrifying to move precious data like that.
I'll definitely be starting a Smartypig for a new machine in about seven years, my guess is that by then, everything will be SSDs. The only downside to this move was losing my ability to write for a few weeks, and pushing back the planned release of HORROR VACUI, but whatevs.