I suppose transaction fees could be assessed and awarded to miners by the same mechanism that currently awards new coins to miners.

But, how are transaction fees to be assessed on a decentralized network?  If someone is just emailing someone else a string of code in exchange for goods or services, who assesses the fee?

I appreciate altruism but do not believe it is something to rely on.

BTC works as a decentralized digital currency because people all over the world verify transactions in exchange for being awarded new bitcoins.  This is known as mining.

Bitcoin creation is capped at 21million bitcoins.  What I do not understand is, why would anyone dedicate computing power to verifying transactions if they will not be awarded bitcoins in exchange for doing so?

It seems to me that once the creation process of bitcoin has ended that the network will cease to function in a way that has currently made it so appealing.

Am I missing something here?  I am excited about bitcoin but less so if it will cease to be useful in a little over 100 years.