It is simple, once all the bitcoins get mined, then the miners will get the fee reward, and that will be enough motivation to keep mining, but the amount of bitcoins reward from each block is not something to worry about, i would say the complexity of bitcoin will be a fact to worry about, right now the total mining power is just nuts, and can't imagine how it will be for 2140, it will be something insane, we will see some kind of evolution step, just as it did, from CPU to GPU to Rigs to ASIC to .