Re outages, it is not about the blockchain itself necessarily, but all the services and systems that interface with it.
Errors occur because humans are humans and fat finger trade details.
That aside, as I mentioned in original comment, instantaneous settlement has other problems, e.g netting is important in managing liquidity.
And to your point about crypto managing billions of dollars of tokens - it's an absolutely trivial amount compared to what's moved in the markets every day. Blockchains are incredibly inefficient and transaction rates orders of magnitude lower than what'd be needed.