I'm still using my 2006 Mac Pro (in my signature).
I have a hackintosh running El Capitan as an iTunes media server supporting 3 Apple TV's and also serving as my primary music station to rock the upstairs (and my poor neighbors across the street).
I have a total investment of about $50 into the hackintosh. Probably less, but being generous in my estimate.
Honestly, it performs very well. And aside for encoding tasks, I'd say it's as responsive as my Mac Pro.
So, if my Mac Pro died, I'd probably replace it with another Hackintosh. I'd probably go a bit higher end than my current Hackintosh. But, I certainly won't pay Apple's markup for computers again. I've been unimpressed with their hardware offerings for years now, and it's just not worth the price to me anymore.
I'm also in a position where I can just as easily move to Windows, and be fine. I actually started migrating to Windows a while back, and have no complaints. The Mac Pro and my Hackintosh are my only remaining Mac OS machines, and I like them fine for the tasks I use them for. But I no longer use Mac OS for production or business purposes.
I do prefer doing my video and audio work in Mac OS. Hence the Mac Pro still having value to me. And I prefer having a direct replacement for my iTunes library if it should become damaged. So the Mac Pro and my Hackintosh maintain identical libraries. So if the worst happens, I just clone the iTunes library from one machine, and the other one is back up and running. No investment lost.
Given the architecture of OS X, it's easier to just keep iTunes in one format, than it is to have a Windows and a Mac library for backups.
A PC would obviously be fine, but generally my PC's are performing real work, while I let the Hackintosh just sit idle waiting for a TV to request a movie, or for a party to break out that requires it to liven up the dance floor with music (yep, the house is equipped lol).
So, my answer would be Hackintosh or a Windows replacement.
No way I'll ever buy another iMac (I've never had one survive my workload - always die cause I push my work machines to their limits, and iMacs can't thermally keep up with what I throw at them).
And there's no way I'll ever pay what Apple seems to think a Mac Pro is worth. I can get what I need much cheaper in a PC. I don't need a Mac Pro. I just need a better box than an iMac.