Thank you, so the fee probably wont increase but the value of btc will?
Hard to say... It'll probably be an equilibrium between the fee size and the diff (influenced by the number of miners).
It's all economics.
Rising fees will push people towards creating less (small) transactions (maybe towards altcoins or offchain transactions)
Rising diff will push miners out of business
Falling block rewards will push miners out of business
Falling bitcoin prices will push miners out of business
The opposite of these situations can also occur, but nobody can say what the equilibrium will be.
Maybe (just maybe), bitcoins will be worth $1.000.000/1
BTC by the time the block reward is close to 0. In this case 0.16
BTC will be a big chunk of money, worth mining. On the other side, bitcoin might be worth $1/1
BTC, in this case the fee will need to rise substantially and there will be many miners that go out of business...