Could they design something new? Sure, with whatever else might be required around that. Currently, however, there is no such thing as a tesla model 3 or Y with an ryzen CPU, without the heatpump, and the 16v li-ion battery, and I believe the octovalve.
They could design something new, or offer a retrofit with the li-ion battery and ryzen CPU if they wanted to, but its fairly rare for any car manufacturers to offer such things, The fact that tesla has even offered anything as a retrofit is pretty amazing, since other car manufacturers tell you "no buy the new one".
Even car manufacturers that say "this car is software updatable", other than Tesla, either release 1-2 updates a year, or none.
Would I buy an upgrade for my 2018 model 3? If it was offered and it wasnt more than a couple grand, yes. Am I expecting one? No. Am I surprised there isnt an upgrade? Not even a little bit. Do I think Tesla owes me the ability to upgrade my 2018 to the ryzen processor? No.
I get the feeling I am in the minority of Tesla owners on this though. As a group, at least as represented online, Tesla has the most entitled car owners I have ever seen.