RE: Bitcoin Mining Apocalypse? Programmer explains.
Interesting hypothetical issue but I don't think it will ever be tested. If miners are forced to have income from transaction fees only, and there are far more users transacting, there's a chance that the difference will be made up and one just shifts from block rewards to transaction fees.
Also any increase in transaction fees would likely be offset by the superior SOV. Hashing difficulty adjustments and Moore's Law will probably have caused mining decentralization by then.
But I think with all the new proposals, Lightning Network, EOS, the Tangle of Iota, Cicada DDD, that in 20 years time this will all be moot.