Blockchain is just an incredibly inefficient distributed database.

Agreed. Most of the time a traditional database is far cheaper and better performing.

The key innovation of blockchain is that write access is algorithmic, not centrally controlled. This feature is not possible with any other technology.

There is no central entity, so it is decentralized in the original definition. I gave Canton as the example because it could be immediately substituted for Swift. It's permissioned access by design, because banks don't want to risk their monopoly position.

For speed and cheapest, go for solana.

For reliability go for Ethereum. For reliability, speed and cheapness choose an L2 on Ethereum.

For illegal stuff use monero.

There are trade offs between them, but I wouldn't call them flaws. Blockchain as a technology definitely "works".