I'm don't think satoshi planned for the 1mb size to be immutable. When Garzik suggested a patch to increase block size, satoshi stated this can be phased in later. I'm not certain the segwit work around was quite what satoshi had in mind.
https://bitcointalk.org/index.php?topic=1347.0What satoshi said doesn't mean what satoshi knew. He may or not have known that 1 MB blocksize would eventually become yet another value which becomes immutable due protocol maturation, what matters is the results, and the situation is clear: Reaching wide consensus to raise the blocksize at this point is basically as controversial as raising the max cap. You can find arguments on both sides, which as a result, the status quo prevails.
I thought segwit was a blocksize increase though. From my understanding, the witness data is still part of the blockchain and legacy nodes are provided a stripped version without the witness data. I don't see that as 1 MB being immutable. I'm sure if the developers can implement this work around to increase block size, there will be other work arounds in the future that increase blocksize.