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.
I think bitcoin transaction fee will be lower and might be even free when the mass adoption occurs and bitcoin transaction fee will looks like be money exchange. I hope that mass adoption occurs earlier than what we have expected so that we can set aside the bitcoin transaction fee because we are losing a lot of money due to transaction fee and it will be a big help if there will be no more transaction fee especially for those who are very active and always doing bitcoin exchange.