Some of them live forever, they might not function anymore though

My logic is something else as I’ll elaborate further.
They will be prematurely outdated.
Given the time it took to present 2019 MacPro and the then touted upgradability which clearly conformed the design, and considering the price tag, I’m just saying it would be nice and smart from Apple to offer a silicon upgrade solution to those who made such a significant investment, considering this:
a) Design cycles of MacPro tend to last 6 years: 1st gen 2006-2013; 2nd gen 2013/2019; 3rd gen 2019/2025?
b) Average 8years cycle for a Mac to became considered obsolete by Apple;
c) June 2022 deadline for AS ARM transition.
So for current version, Apple has two choices: doesn’t do anything until a redesign or offers some update/upgrade options.
Given the redesign cycles, I believe they will opt for second choice. At this stage it would feel odd to offer Intel x84 solutions which would increase the lifetime of “dual ARM/x84)” versions of MacOS and consequently would compromise the “natural” obsolescence of Intel Macs.
Since the current built, being so “generous” and flexible, doesn’t prevent to get a new SoC in, this could be easily done.
However all this doesn’t mean excluding a new smaller chassis design.In fact I wish it happens soon, in the form of a new “cube” as some rumors suggest. If new affordable 27” 5K Apple Display also comes true, I believe the bundle would be catering a lot of needs and desires of the Mac people: those longing for an upgradable MacMini Pro; those wanting a more affordable MacPro Mini and I might add those wanting an iMac Pro refreshing.
All this would allow for a redesign of top of line MacPro within usual timeframe, while keeping transition deadline, and equally important without compromising or diminishing the distinctive upgradable architecture of the MacPro and I would add it’s durability, which helps to justify the heavy investment. Not p*****g off customers would be a bonus...