This is the chart of Bitcoin's Avg. Transaction fee:
https://bitinfocharts.com/comparison/bitcoin-transactionfees.htmlRight now the median fee is about $2. After lots of halvings the reward won't be enough to satisfy the miners. I guess that after a decade, bitcoin will be more widespread, as a result, more transactions will occur every second. Won't this increase the median fee? If it does, bitcoin will start being a not-so medium of exchange. Imagine paying $5 for a transaction. Even paypal doesn't charge that much.
The increase or decrease in Bitcoin transaction fees is dependent on the demand for Bitcoin. If the demand starts to decrease, the fees will start increasing. If the other way around happens, the fees will be start decreasing. So, demand is a very important factor on this.