There are plenty of contexts where writing in a single flat line is necessary, so it's still useful to address the issue.

Just using more brackets is always a solution, but it can become messy and hard to read if you take it to the extreme (there's a Minute Physics video where he does this and it unintentionally shows you just how bad it is), so it eventually becomes a matter of agreeing on convention and using brackets judicially where there's actual ambiguity.

Usually you’d use a fraction bar, which groups it and makes it unambiguous

Division (operator) and fraction bar (grouping symbol) aren't the same. It already is unambiguous.