I think the issue will be that either your Mac will be unsupported for OSX Mojave or at most in an year or so while your machine physically will still be in good working condition.
I think the only issue will be if I need to use XCode, which tends to be bound to the latest OS.
What precedent do you have for believing that to be true? The only time Apple has done that in the past is major platform changes (aka PowerPC and later 32bit CoreDuos). Unless Apple shifts to ARM (which is easily a couple years out at best) or Intel comes out with a 128bit processor (which isn’t even remotely on the roadmap), I’d say you are safe for at least another five years, even if those shifts do happen.
There are aftermarket GPUs available, but it’s becoming an ongoing chore to keep the machine running the latest macOS features (needed new wifi card to support handoff and watch unlock etc).
New wifi, SSD, usb3 etc are relatively cheap. The new GPU is getting more expensive and is now actually blocking keeping up to date with major OS releases.
There is no other Mac I currently want to buy (desktop, my own display). MacMini is too underspecced, MacProvis about to be updated and aside from GPU isn’t really any better than my current MacPro - feels end of life and outdated from day 1, a non-starter for me.
So I’ll probably get a new GPU and hope that buys me another couple of years.
I've put in USB3, replaced the spinning disk with SSD, upgraded the WiFi and RAM, but haven't done the CPUs yet (already got dual 6 cores, so aside from a slight clock bump not much point).
My only concern is that the CPUs don't support something that the next major version will require, forcing a move off the MacPro 5,2 platform.
I also share your concern too. :(
I got the kit from OSX wifi:
http://osxwifi.com/apple-broadcom-bcm94360cd-802-11-a-b-g-n-...
Works well, had a bit of trouble getting unlock with watch working, but then all of a sudden started, so not sure what that was about, but been good since then.