Yes, let's design the internet around keeping 90s routers running as long as possible. Excellent. Let's also design highways with a max speed of 20 MPH so we can accommodate Model T's
Slowly over time the roads were upgraded, so now you can use mostly high speed roads to get between places and you don't really see Model Ts anymore because it's not convenient to avoid all the high speed roads.
Nothing has stopped making faster cars just as nothing (fundamentally) has stopped us from making a "better" Internet (supporting more addresses and other features). What hasn't been done is to say, "Ok, so we already have this massive hardware rollout but we're going to literally obsolete 99.99% of it overnight and cost everyone, home users, small businesses, governments, megacorps, billions or trillions to replace their hardware."
Why didn't that happen? Because it would've been stupid. Instead we ended up with what's amounted to two Internets with bridges between them. The transition hasn't been smooth, but it has been happening.
Attempting to change the format of an existing protocol in-place in a way that existing hardware and software cannot handle, would be an effort that's dead on arrival.
The global transition to ipv6 is already hard enough, and that's with it designed as a separate protocol so that existing equipment that doesn't understand it can at least gracefully discard it instead of mishandling it.