Well, I do wonder how these dashes are inserted in the first place. An n-dash or m-dash does usually not exist at all on any normal keyboard, right? So, maybe it is a certain app or automated import that is causing this. If this is the case, it would be more meaningful to root out the source of that.
The obvious downside of a mass edit is that the last-edit date of the elements are also updated, causing those elements to appear as if they are up-to-date to software that evaluates that (like StreetComplete) even if they are not.
Also, there are many more opening hours strings that are invalid according to the spec but still unambiguous enough that a lenient parser can understand them. From the numbers above, this would be about 100,000 opening hours. So, your edit would just fix less than 1% of these invalid but unambiguous opening hours.