Assuming the protocol doesn't change, there is currently a 1 megabyte maximum size for a block.
Such assumption is not reasonable.
Will it always be 1 MB / block - can this be changed? Will this suffice to process all transactions in future?
Surely it can change, and likely will.
It's also likely that there will be people that will insist in remain in the "old version" where no more than 1Mb blocks can exist. That will provoke a fork. This will provoke lots of turbulence in the short run. In the long run, the scalable Bitcoin will "win".