How will it end? Not with a bang, but with a lot of whimpering. ;-)
The question is very poorly stated. It ought to be, "When will the newest macOS no longer support cMP?"
As others have noted, security patches for an OS tend to stretch 2-3 years beyond the introduction of the next OS, so if support for cMP ends with Mojave, we're looking at 2-3 years more of "cMP support" after that. If cMP is still supported on 10.15.x, it's an even longer reprieve.
As others have noted, official Vintage/Obsolete status follows a strict clock that is unrelated to hardware or software viability. The only criterion is time from date of last manufacture. OS support is more variable, and is usually tied to the inability of the hardware to support features that Apple decides are essential in the newest OS. Since peripheral chipsets tend to be present in all Mac models of a given vintage, regardless of CPU model, "It's not personal, cMP."
A typical example is requiring that the Bluetooth, USB, or Wi-Fi chipset supports a particular revision level or protocol (Bluetooth 4, USB 2.0, etc.). There's not a simple, "All Macs made in 2012" cutoff because the product rollout schedule is somewhat ragged - there's no "12 months between new models" rule, as both Mini and nMP fans know all too well.
So, of the reasons proposed in the survey, here's my call:
Lack of AVX or other Intel instructions - seems the most likely possibility. AVX was introduced with 2011 Sandy Bridge processors. That would obsolete anything older than early-2011 MBPs, mid-2011 iMacs, Minis, and MBAs; early-2015 MBs, and late-2013 nMPs. In other words, lack of AVX would doom every cMP.
Lack of T1 / Apple In-House Co-Processor - really really unlikely, as requiring T1/T2 would obsolete all but the Touch Bar MBPs and iMac Pro. First, get a T1-equivalent into every new Mac model, then add at least five years. The cMP will likely lose macOS support long before 2024/2025, which I figure would be the earliest they'd pull the plug on non-T1/secure enclave models.
Switch to Apple In-House CPU's - really, really, really unlikely. Apple provided dual support (PowerPC and Intel) for Tiger and Leopard. Tiger 10.4 was released in April 2005 and continued to receive updates until September 2007 (10.4.11). Leopard was introduced in September 2007 and received its last update (10.5.8) in August 2009. Since the last PowerPC Macs were introduced in late 2005, that meant they were still getting new OS updates nearly four years after the first Intel Macs hit the street. Even if Apple switched to A-series processors in 2019 (highly unlikely, based on the lead-time developers need to make that kind of switch), it could be four more years before they stopped updating Intel-compatible versions of macOS. Again, with today's cMPs already on the cusp of non-support, this isn't going to be the straw that breaks this camel's back.
cMP's won't lose support because Apple uses them internally for development - They use them for development? Not compatibility testing, but new product development? You know this for a fact? It seems awfully unlikely, unless certain individuals have enough institutional power to resist the replacement of a beloved bit of hardware. They're certainly not going to be handing cMPs out to new hires. Considering corporate computer purchases are fully depreciated after four years, and Apple is not so poor that they need operate old hardware past its normal expiration date... No. Given a choice between nursing along old cMPs (including upgrading GPUs to support Mojave) and issuing a brand-new or refurbished iMac Pro, what do you think corporate IT wants to do?
Lack of iMessage / iCloud services - Huh? How old is the version of OS X running on your cMP? If you mean support for Animoji, Memoji, or whatever moji come next... what Mac supports any of them? Are they going to obsolete every Mac built before 2019? What iCloud service is likely to come along that demands so much of hardware that a cMP couldn't hack it? All I can think of, cloud-wise, is ratcheting up security protocol requirements for Wi-Fi. WPA3 was just introduced earlier this year. Afaik, no Mac supports it. I predict it'd be a minimum of 3-5 years before someone like Apple would shut off access to their ever-more-profitable cloud services to customers who have failed to upgrade BOTH their Macs and their routers (iOS devices are generally replaced on a shorter timeline, of course). Again, cMP is going to lose new-OS support much sooner than that.
Bottom line for me? I'd vote for None of the Above. If past history is any indicator of future performance, I expect it'll happen within the next couple of years, but the reason will be something completely different. Nobody expects the Spanish Inquisition!
(Two Python catch-phrases in one paragraph? How dare I!)