Remember the MacBook Pro with a DVD drive lived on for years after all its siblings were discontinued. We can expect some x86 support to remain available for years after the last new ARM machine is launched.
I think that's the mid-2012 MBP? They were still selling them in 2015, I bought one of the last ones before it was pulled from sale. It remained in the lineup because it continued to be a best-seller.
Apple was already grumpy about servicing it by 2018 or so though, even with AppleCare. (I've switched to Windows now.)
I think it could be tied to large contracts and ensured availability. I suspect there will be an x86-based Mac available well past the two year transition that was announced.
Much like the 2015 Macbook Pro I expect you'll be able sell it for more than you paid for it years later.