Something that was created to be an alternative can be a challenge to others if people find it more better than what has been the traditional thing. I can comfortably send someone in other part of the world Bitcoin right now and they can receive it under some minutes, sell it for cash without raising a voice but if you want to do wire transfer today, it might take 3-7 days before the recipient will see access to that money, now tell me why banks wouldn't envy Bitcoin.
Bitcoin is what we should have had from beginning before all this fiat system that is controlled by the government, Bitcoin doesn't have anyone who control it. Do you know that today if government want to have Bitcoin as official and regulate it to their own taste, they can do it and even minimize somethings they fear about like money laundering and the rest. It will even be better than the fiat system they think is better than Bitcoin they talk about all the time.
Cross-border money transfers with faster times and cheaper fees are one advantage of bitcoin over banks or the traditional monetary system. But is that attractive enough for people to switch to bitcoin and abandon the banking system? What do you do if you accidentally send to the wrong address or the recipient accidentally gives you the wrong wallet address? Can a transaction be reversed? Or bitcoin is volatile, what will you do after completing a transaction and bitcoin price drops 5-10%?...You won't have those problems with the banking system and that's why large value transactions are still processed through the banking system instead of using bitcoin, so far.
Also, does bitcoin provide financial solutions and services like loans, financial consulting, insurance...like banks? How can it challenge the banking industry just because it has one advantage of faster and cheaper international money transfers? while it can barely provide any of the critical financial services, which play an essential role in a country's economy.