I think you make some very valid points. To be clear my view isn't that blockchain is going to make banks obsolete (this blog post sums that up pretty well: https://blog.chain.com/a-letter-to-jamie-dimon-de89d417cb80). For example, certain activities that you mention, like M&A, are not going to be easily improved by just introducing a blockchain.

What a blockchain is useful for is eliminating the need for a centralized, trusted third party. Where I could see this being very helpful, and a potentially disruptive force is in areas like IPOs, mitigating counterparty risk, reducing conflict of interests, etc.

For example, if companies can raise capital and create liquidity for themselves with ICOs, the demand for IPOs will decrease. If investment banks break the "Chinese" wall and engage in market manipulation, then having public companies put their accounting on a public ledger will reduce the dependence on quarterly earnings reports. If more players are able to access the financial systems because they are on public ledgers rather than private ones, it will make it harder to banks to engage in oligopolistic behavior.

None of these things are guaranteed to happen, and certainly they won't all happen overnight. I do see though, the potential for these types of changes to slowly chip away at some of the revenue streams that banks have enjoyed for a long time.