bitcoin would handle 24k*60*10=14.4M transactions per average block. Also, if every transaction is going to cost, let's say 0.01$, it would mean that every block reward would be 14.4M/100=144k$ which is around twice more than the miners receive from blocks right now (78k$). It's a problem that will solve itself with time and we don't have to worry about it.
Thanks for such a great insight about profits. I am pretty sure transactions will cost a lot more and the profits will thus be even higher, but for the success will also need there transactions to happen. If people keep storing bitcoin and rarely buying and selling some, miners won't get enough transactions to confirming for their power usage to be compensated well. The community will be of essense at that time, I think.
When 21 million Bitcoins are mined, the second source of income will be gone. So how will the bitcoin network do when there are no miners? I do not think that with only a single source of income, it may be profitable for them when the Bitcoin price goes down.
Apart from transaction fees which were explained by buyinbtc, miners can always turn to other coins. I am pretty sure there will be enough to mine. Also, I am not sure it's technically possible with bitcoin (but since it is with ethereum, it gives hope), bitcoin can eventually become a Proof of Stake coin and those who mined btc will be able to profit from holding it.