Apple dropped everything at 5 years for Ventura, so that's as long as I would count on being able to run things with full patch support.  If you're right and the Mac Pro drops off at WWDC, then 2028's WWDC is the furthest out for the of the line - question is, will they drop intel from the OS and as you suggest, just extend security patches for that last intel version for a couple of years.  Wouldn't surprise me to see that happen with the OS release in 26 or 27, and an Mx only release at that point.  I think 26-28 is a fair guess for when folks will be forced to update intel hardware.  Remind me to buy stock that next year