If that happens just imagine the fees you have to input for every single transactions. Logically if there is a solution where you can input more transactions in a block then the miners could have a profitable time when the mining reward ends, if not no one will mine if there is no rewards and you cannot survive with fees alone if the number of transaction does not increase. There is another hope in this regard, when the mining will not reap much profit, all the big miners will leave the scene and then home mining will be possible if anyone is willing to support mining and as a reward you will get the mining fees.