I pretty much agree with @codafi's position (though @codafi feel free to correct me if this seems mischaracterized) that
_.self is, at best, only marginally more inscrutable than similar forms such as
(_, _) or
(_) -> _. Already, there are undoubtedly situations where it's 'obvious' that the specified type is going to be a two-element tuple, or a function of a single argument, but the element types/parameter and result types are non-obvious. The restriction on top-level placeholders only saves us from one specific issue, where the top-level type itself provides all the context needed.
As you note, we can already write
Some<_, _> (or even without SE-0315, just
Some), so it's not clear to me that allowing one more step of inference makes the difference between "clear because the type was specified in source" and "extremely inscrutable."