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.
I agree fully with the above but unfortunately the 1MB block size has already been abandoned when segwit was introduced. This wasn't a prerequisite of segwit at all so it's a real shame and a real danger of having set a precedent (at least in the minds of some) for another increase in the future.
I hope there will never be another increase as every increase exponentially reduces the effective security of the network.
Segwit was a soft fork not a hard fork. Nothing was abandoned as its all 100% backwards compatible