1. Decreased in transactions: this is simple and self-explanatory of bitcoin becoming the main source of higher transactions in the future. The traditional banks will have a reduction in people that transact with them, and that is capable of causing them low revenue for the banks, which will make it very hard for them to sustain and run the banks.
Bitcoin has its cap in block size and maximum transactions per block and if Bitcoin protocol won't have upgrade, it will not be able to handle massive new demands from bank users who switch from bank systems to Bitcoin blockchain.
Bitcoin will be competed by altcoin blockchains with cheaper transaction fees and higher TPS (transactions per second). I love Bitcoin but this fact can not be denied. Bitcoin takes out customers from bank systems but altcoins will take many users from Bitcoin too. For transactions, they will choose what gives they cheaper transaction fees if the transaction value is not big enough, to force them thinking of security and risk of reversibility of their transactions.
2. Security and fraud from scammers: traditional banks will be in danger from scammers, so they have to increase their security measures in order to be protected against some bitcoin-related fraud because bitcoin is something that requires knowledge, and many scammers are trying to use this opportunity in other for them to scam other people who are willing to join.
Scammers can scam in both bank system and Bitcoin blockchain or altcoin blockchains. It's wrong to say they only can scam in bank system and can not do the same with Bitcoin blockchain or altcoin blockchains. People who lack of knowledge or are careless will be more easily scammed with any system.
Bitcoin can not do everything, replace bank system, fix economic problems, beat scammers, it can not be a magic tool to do everything.