Probably the best thing that could be done overall is Google's already-complete plan to move the web browser, and as much other code as they can, to Play store apps that they can centrally auto-update. But getting that solution onto the vulnerable phones required them to, once again, just upgrade to the newest Android.
I can probably manage to flash it, but the typically phone user probably shouldn't be attempting that. "Just upgrade" is not a valid response when they refuse to push the upgrade to all users.
I have an old Galaxy S3 that Samsung no longer updates, yet these bugs don't effect me because I've replaced the built in browser with Chrome, which still receives regular security updates. What's stopping users of old phones from installing Chrome or Firefox?
Note: I don't.