how certain are you that WPA3 can’t be shipped as a software upgrade?
I'm pretty sure that the wifi devices I own that were EOL'ed a decade ago aren't going to get a software upgrade. Nor are the ones from companies that are no longer in business.
Is everything you own EOLed? If your router is that old you’re probably open to security issues and definitely leaving performance on the floor.
For most people, I’m expecting primary devices (phones, laptops) to get updates along with newer routers (at least the higher end ones).
The big question is how common the practice of segregating old devices will be since it’s still easy to find things which don’t even do 5GHz and that includes a majority of TVs, Android devices, etc.
I'm pretty sure that the wifi devices I own that were EOL'ed a decade ago aren't going to get a software upgrade. Nor are the ones from companies that are no longer in business.