Isn't there a little bit of 'chicken-and-egg' here?
Why would miners use these new 'efficient schemes'
in the first place if it was to render the fee market
ineffectual, unless they were already relying on something else
(blocksize limit) to do that (support a fee market)?
These schemes reduce orphaning and allow miners to make a greater profit. It's not hypothetical, Matt's fast block relay protocol is the simplest and least effective of this class of scheme and it is used almost universally today to carry blocks between pools. (A good thing too: otherwise there likely would only be a single mining pool now...).
I don't understand. If its 'used almost universally' then how can it be a competitive advantage that would result in higher profits?
Also whatever the miners are doing today seems like completely different scenario (block subsidies are still high and there is also a blocksize limit).