I still have a 2008 Mac Pro (tower). At the time it was possible to order the bottom of the line system and add your own memory, video and drives, just like a "normal" PC. I think it cost about $3000 new which was still the most I'd ever paid for a computer. Picked it up from an Apple store. I used it as a home workstation for work and music and a gaming rig by dual booting to windows -- which no longer works. Did a few upgrades, video card, SSD, more memory. It's a dual quad core Xeon, 32gb of ram and 2tb of storage. It's almost 15 years old now and it still runs fine.
Except!
I feel like there's been a lot of forced obsolescence for sure. Things really went downhill like 2-3 years ago. Gradual lack of OS and driver support made it harder to use as a music machine. Boot camp was no longer supported without some crazy firmware hacks that I didn't bother with. I just got an xbox/ps instead. After some combination of OS and driver updates, my firewire audio devices no longer worked. The USB Audio devices I tried were always terrible (keeping the machine from sleeping properly).
It can still mostly log in to iCloud and iMessage, but there are weirdnesses. I did pick up a used firewire interface that does work with this OS (El Capitan).
But I really think the hardware could be made to work on the latest OS with very little effort.