Yes, any one of these parameters could be changed with consensus.
My point is exactly that they can't, as long as we have a distributed, trustless system. It can only happen, if there is sufficient centralization.
Would you accept that suddenly, the block reward jumps to 500 coins because this suits miners ? Do you really think this is possible by consensus ?
Most miners are not short sighted evil entities. They would rather process more transactions with lower fees as this increases the utility value of what they are mining.
Of course they are "evil entities" like all of us. All entities in a trustless system are to be considered as evil entities. Otherwise, if they are to be trusted, the system is not trustless.
The point is, however, that there are *different ways* to increase the number of transactions, and that there will not be an agreement on which way, because that will influence the gains and the losses of some. One will not come to an agreement, even if an agreement is beneficial. There is no way to come to a consensus, as there will always be a sizeable fraction that doesn't want it that way but the other way.
You see it with segwit vs BU.
