The immutability of the 1MB blocksize still stands if we throw out the word "protocol". Satoshi knew that once he slipped that blocksize limit into the core client, it would be there forever, and become the de facto standard in all bitcoin software implementations. That's why Garzik and the others were opposed to it at the time. Satoshi said that it was temporary, but he knew enough about game theory to realize 1MB blocks would become a Schelling Point within the bitcoin network if it was left in the core client long enough.