As far as lifespan goes. The Mac Pro 5,1 may have a longer useful life. If you like tinkering. As you can upgrade the GPU you can keep running new versions of macOS with the assistance of patches like from DOSDude. Even though 6,1 has longer official support. Once a new version kills GPU compatibility. That is the end of the line.
This has some huge assumptions in it to try to push a huge gap between the two when there pragmatically isn't.
First, that GPU is only major line of changes. The x86_64 instruction set is changing over time. The 5,1 is just as 'stuck in time' as the 6,1 in this respect. Actually even more so since it has an even older set of x86_64 instructions. If a future macOS instance pragmatically says AVX2 is required the 5,1 is done. When the other Mac models that also don't have AVX2 fade away, it will be consistent to drop the others too. Same thing with virtual memory and the other instruction subset areas that are still getting dynamic changes.
Drivers for elements embedded in the chipset ( USB , Ethernet, etc.) same issue.
The "immunity" of the 5,1 because can swap out GPUs cards is rather myopic. It gave the 5,1 an incrementally longer runway to extensive desupport. ( quick peak at the DOSDude website has the MP 1,1-2,1 on the not supported list). Over ten years other stuff changed than just GPU tech.
Periodically, Apple does 'Spring cleaning' For example.
"...macOS frameworks are now thinned for the x86-64 architecture. ..."
https://developer.apple.com/documen...tes/macos_catalina_10_15_beta_5_release_notes
periodically they will chuck "old stuff" that has been on the obsolete and deprecated list for an extended period of time.
Second, it is a major assumption is that Apple isn't going to further limit rouge installers and rogue core system components. Rogue kexts , modifying Apple's core system code, etc. are all probably going to get harder to do over time. 10.15 Calalina is bringing a split of the system file store into a different file volume than than user data/code and applications. That is probably "step 1". A non Apple system installer ( all the way up to the system content being installed) may not work in the future. Installers for the system and applications ( and new system extensions (new style for kext) ) could be on two different tracks in the future as they are on separate volumes with different security.
Not being on Apple's Obsolete list is more likely path to future system support than being on it. The 5,1 has a bit of an edge for users who want to snapshot on the currently available systems and do production work in a time bubble for as long as possible.