The intention between the warnings is different though: one is supposed to help beginners understand that they are doing something which they don’t want to do, while -Ywarn-value-discard will emit a warning everywhere a value is discarded, which I suspect makes a lot of people not use it.
Ideally, this kind of code would not compile at all, but I suspect it won’t be easy to change unit inference to uniformly achieve that. At least, I’m pretty sure I’ll be able to implement the proposed warning (which is part of why I’m proposing this), but changes to unit inference are a whole different game.