Why do you say they'd have to be splattered over basically all rust code? The post compares them to Send and Sync, and I don't have to use those very frequently (though I don't write a lot of rust code, either).

People use Arc and Rc all the time, which would need +?Leak annotations. Things like Option would, I think, also need + ?Move + ?Leak annotations.

If it is an unobtrusive as the existing ?Trait annotations such as ?Sized it's not very much of a burden.

Because features are always used in the worst (most evil?) way possible by inexperienced programmers. One of the benefits of Rust is that it makes many evil things either impossible or easily detectable.