How much will be the fee after all bitcoins are mined? Because mining is every time harder, and hence more expensive, right now is profitable because the btc is rising, but what will happen when btc price is stable and miners won't get any extra reward, only the fee.
If the fee is high then we will end in a worst scenario than current transactions with banks :(
Is there any approximation about this fee? (In btc because we won't know the price in usd)

EDIT
similar question (but too old, without answer):
How much will transaction fees eventually be?
Maybe this could be a good approximation
How many BTC are transferred per day (daily volume)?: 750.000 BTC (source: https://coinmarketcap.com/currencies/bitcoin)
How many BTC are mined (generated as reward) every day?:
1800 BTC (source: https://www.bitcoinblockhalf.com/)
How much is paid in fee every day?: 34 BTC (source: https://blockchain.info/stats)   

So right now the fee is 34 / 750.000 = 0.0045%
But if we mined everything, then we need to pay the 1800BTC to miners in fees to compensate so will be (1800+34) / 750.000 = 0.25%
If this is the case I think is cheaper than current bank transactions fees... 

But I'm not sure if this is a valid approximation, fee seems to change a lot (not sure why or even how to read this graph either): https://bitcoinfees.info/
As fees for transactions are a free market and supply is limited (due to the block weight/size limit), the price will be whatever people are willing to pay. Those who pay less than the market rate will see their transactions slowed down or ignored; those who pay more will see them processed quickly.

What is not known is whether the resulting fees paid will be sufficient to keep the network secure. As security against chain rewriting is proportional to the cost spent by miners, we can expect that it is also proportional to how much miners are paid. If people are not willing to pay high fees at all, the security may collapse in a tragedy of the commons. If people do, and the value secured by the network is high, fees may grow very high as well.

What Bitcoin (the blockchain) technology offers you is an independent currency, beyond control of banks or nations, which permits you to autonomously audit the entire system. While you don't have to, it permits you to be your own bank and be entirely independent from third parties.

What it does not offer directly is a convenient payment system. Bitcoin transactions are slow, inconvenient, inefficient, and unreliable - and perhaps expensive too. They are pretty much the opposite of what a consumer would want as a payment system.

However, due to being a natively digital currency, without third parties that can deny you access to the system, the hope is that it can be used as a base layer in a financial system upon which actual payment technology can be built (sometimes called "layer two"). A very promising proposal that is being actively worked on is Lightning. Lightning offers cheap, reliable, fast payments, on top of the Bitcoin chain. However, not every Lightning payment corresponds to a transaction on-chain. Yet, its security is very similar to the blockchain itself and still does not rely on third parties.