The brick wall thing seems like a particularly silly analogy. People are already competing for block space with fees. Why are people doing that when block space is effectively infinite? Why are miners encouraging that when the fees are so meaningless in comparison to the subsidy? It seems like the car hit a wall of jello a couple of miles before hitting the brick wall.
The brick wall is analogy is accurate because It's the difference between being able to send transactions and not being able to send transactions. Right now all the users who want to send Bitcoins can do so because the total demand is lower than the protocol limit.
Once the blocks fill up we'll get to a situation that changes. If more than 5000 people want to send a transaction in the same 10 minute interval they won't all be allowed to do so, no matter how much they are all willing to pay in fees. The ability to use Bitcoin will be rationed to a fixed number of entities.
What I want to know is, how much cheaper? How well will people innovate? Are people willing to pay enough in fees to keep mining going as the subsidy declines? What is the actual marginal cost of including a transaction? How will that change over time?
There are a lot of question marks between where we are and where we want to be. It seems imprudent to assume that they will all be resolved to our liking.
It's complete economic and historical ignorance to assume the best way to answer any of those questions is with central planning in the form of transaction rationing.
Miners will mine if they can obtain a market price for their services that is acceptable to them. Users will initiate transactions if the price if sending transactions is acceptable to them. Not only is there no need to second guess price discovery by trying to calculate the marginal cost of handling a transactions and using that as the basis of a magic number in the protocol but also succeeding at such a strategy is
probably impossible.