My Mac Pro 1,1 lasted me from Tiger to Leopard to Snow Leopard to Lion to Mountain Lion to Mavericks to Yosemite to El Capitan.
Why wouldn't a Mac Pro 5,1 stop any time soon?? It has a 64bit EFI (32bit EFI was the reason Mac Pro 1,1 and 1,2 were left behind at Lion)
I'd say having a Mac Pro 5,1 is the last truly great machine from Apple that's fully upgradeable.
They're just old and inherently with older machines you experience A LOT of problems. While I love my Mac Pros too, I don't use them as my main workstations anymore. They're great machines for the short to mid term but as a long term work horse machine I think not. Fully upgradable doesn't change the fact that it's going towards fully obsolesce.
In the next few years 4,1/5,1s will begin experience the "worn out" period. The caps, wiring, traces and others will begin to show their age. Sure, you could fix it... but how long do you want to drop money into it? They're remarkable machines and at the moment flawless in OSX, giving you OSX simplicity. As soon as official support drops however, you get into the arena of really complicated maintenance even worse so than Windows. Summarizing, they're getting old and they'll only go so far. Therefore making a long term recommendation bad.
While you may be using yours for a typical home use I'll assume you like most of us using it for professional usage. Apple hasn't released jack while HP and Dell have been regularly releasing and offer insane amounts of customization at a reasonable cost. You can buy something that that can destroy even the most upgraded Mac Pro 4,1/5,1 still at a reasonable price. A lot of people here have bought those in order to stay competitive in the film/video industry... they just can't handle the dynamics of 4k footage. They just aren't competitive now, the general time frame is 2-3 years for a new professional machine.
It's unfortunate that Apple will kill support, but it's inevitable. If your insistent on real Apple macs only then this is your best bet. A well built high end dual socket Hackintosh is possible and doable. I've included an example build from 2015... you probably could upgrade it. If you're fully satisfied by your 4,1/5,1 then stick with it. If you, like me, desire more performance, you can build a reasonable Hackintosh. While the 5,1 is pretty easy right now to run the latest OS, when official support is dropped it would be just as tough as a Hackintosh to maintain. I hope you like this build... it's what I built.
Lian Li PC-A71F-B Case $209
Supermicro X10DAX WS Mobo $440
Xeon E5-2698v3 2.3GHz 16 core x 2 $6498
Dual GeForce GTX 780 $1500
Crucial ECC 64GB PC4-2133 4 x 16GB $785
Samsung 850 Pro 512GB x 2 $678
EVGA 1300W PSU $195
TOTALS: $10305
You would probably want to upgrade this. Even something at half of this price would still crush the 12 core Mac Pro.