This chart brings up a very good point. Like you point out, in such a scenario, the miner can only earn a profit if he can include a sufficient number of fee-paying TXs in his block. But what happens if the miner before him cleaned out the mempool? It seems the rational miner will stop hashing1 entirely until enough new transactions have built up to push his block into the "profitable" zone in your graph (the zone where the supply curve is below the demand curve).
Maybe I'm missing something, please correct me if I'm wrong.
In the event of an empty mempool (and with a 0 btc subsidy), for a miner to obtain a profit would be necesary to wait until enough new fee paying transactions enter into the mempool, otherwise keep mining at a loss.
Isn't this the same as saying
"In order for a transaction to be added to the blockchain, a sufficient fee must be provided"