He didn't make an 'assumption' like those ones you described.

This is an artificial example of a function copying unmodified data from source to destination. There are async and await tokens in every line, because every line is doing an IO operation. I a real world app this data would be somehow processed in between, using synchronous function calls, therefore without async/await tokens.

>most C++ developers don't think anybody will (ab)use 'const_cast', and so on.

These constructs are opt-in. If you don't want them in your codebase you can find their location by a simple text based search and remove them. In C everything is "unsafe". You can't opt-out.

I don't think anyone is saying that you'll never see crates with bad usage of unsafe. What you will hear them say is that by having the ability to share code, since more people are looking at and using the same codebase, it's more likely issues will be found, and that when they're fixed, they help everyone using the package, rather than just those who found it.