I think the categorization of protocol changes above would need to be re-evaluated at some point. I believe that PoW scheme is more fundamental to Bitcoin, than any other aspect of the system, for those additional degrees-of-freedom it provides for the competition (this has been discussed a lot recently).
However, the block size limit, the tx fees structure and even the infamous 21m limit is where it gets interesting. These things are actually inter-related and not completely orthogonal. If it happens that fees are not able to provide enough incentives in the future to keep the network secure, then introduction of permanent limited inflation would be one of the options on the table, no matter how unpopular it might sound today.
Some balance would need to be found between the costs of transactions for active economic participants and Bitcoin's ability to maintain value throughout time for savers. The presence of other similar systems in the environment would not allow one system to swing too much in either direction. If transactions are too costly, another system with higher block subsidy will provide less friction for the commerce, if inflation is too high then another system will gain more traction as a store of value.
This will be very interesting to see unfolding and I would expect a lot more debates in the future for finding the right balance for Bitcoin. People will learn economics by working it out on a live system with their efforts rewarded in terms of growing adoption and increased monetary value. There will be winners, there will be losers, but everyone is given a chance to participate and make their choices, that's invaluable.
PoW will be unlikely to ever be eliminated because of vested interests, so we may as well not waste each others time even considering switching bitcoin from PoW to PoS. Adding additional algos like TaPoS ontop of PoW is certainly possible and may even be carried out at the sidechain or wallet level.
Changing the 21 million limit while, hypothetically possible, should remain in the unanimous consent category as it is a fundamental economic principle behind bitcoin (being disinflationary vs inflationary) and would be unethical to betray the a majority or minority by stealing wealth from them through unforeseen inflation. We already have plenty of examples and choices with inflationary currencies, we are just asking for one disinflationary choice that does not betray the users in this economic experiment.
There are many other ways to pay or accomplish network security and the fact that you immediately are open to completely changing the economics of bitcoin without exploring the other options is frightening.
With this thread , changing the block size , while does have some unforeseen and unexplored economic consequences, doesn't change the core principle that bitcoin remains disinflationary.