Synchronous I/O by default means you're wasting your CPU cores.
But that's the thing: synchronous code doesn't mean synchronous execution. The language/platform should be able to make it async for you, without forcing you to re-shape your code. And it's not a rare system: even plain old threads work like that - in fact, they're often a good solution: https://www.mailinator.com/tymaPaulMultithreaded.pdf
The code itself is not the problem.
Data, specifically mutable state and dependencies are.
Relying on the platform to figure those out for efficient async execution hasn't worked out so well in the past...
But that's the thing: synchronous code doesn't mean synchronous execution. The language/platform should be able to make it async for you, without forcing you to re-shape your code. And it's not a rare system: even plain old threads work like that - in fact, they're often a good solution: https://www.mailinator.com/tymaPaulMultithreaded.pdf