Bitcoin's maximum throughput is about 7 transactions per second. Superior blockchain implementations can generally achieve ~30 transactions per second for long term, effective operation.
Nasdaq's daily trades would require over 430 transactions per second. There are tradeoffs to be made (such as block size or integrity), but it's fundamentally not feasible to port our existing centralized ledger to a decentralized blockchain ledger.
This is just Nasdaq. Imagine trying to scale a decentralized blockchain to multiple markets, or to Visa's requirements, which are over 2000 transactions per second.
Your numbers are nonsense. A blockchain can be made to handle any number of transactions per second. It's simply a matter of increasing the number of transactions per block, the frequency of the blocks or both. The trade off is in the increasing storage requirements. The Bitcoin blockchain currently sits at just over 100GiB. This is an insignificant amount of data for a bank. Furthermore, you can reduce the blockchain storage requirements through checkpointing. Every so many blocks the participants in the blockchain can agree to and cryptographically sign the latest balance sheet. All previous transactions can then be archived. IMO, the Bitcoin Core developers shouldn't be focusing on anything but checkpointing right now.
Do you have any evidence to support what you're saying? The research I cited directly contradicts your claim.
You cannot scale up blockchains arbitrarily without making fundamental tradeoffs to increase their performance (for example, increasingly centralizing it).
Increasing the block size may indeed make a coin more centralised but we are talking about using cryptocurrency to replace outdated systems among banks, government agencies and brokers. In this case, it's not unreasonable to assume that the participants could afford to dedicate sufficient storage space to handle high transaction rates. There is no fixed limit on the transaction rate.
There are so many implementations. Many of these can process orders of magnitude more transactions than Bitcoin.