IF there would be no coins, other members said here the exact answer which miners would still continue to make profit regarding on transaction fees and also bitcoin circulation would be surely moving and if the supply is already limited then expect on the bitcoins price would surely rose up high and would be more valuable but just as expected coins would be mined totally in a matter of time and we cant really see it.
When we gonna see the meaning of supply and demand law we could surely say that bitcoins price would surely rose up in the future when there were no more coins to mine and as you said they could still able to gain money in every transaction that have been made because of transaction fees. Theres nothing to worry about this thing because the last bitcoin would be mine wont come in our lifetime.