Bitcoin (or maybe just bitcoind) would be quickly out-competed if that rule was adopted. Many protocol changes have been made (Satoshi probably made dozens of them after 0.1) and many more will be necessary to keep Bitcoin secure and scalable.

Satoshi seemingly had a policy that clients should work (maybe with less security) for at least two years after they were released. I think this is a good goal.

Ultraprune is not a protocol change, so this proposed rule would have no effect on that.