Feels like pre-1900 sweetness levels of wine in general is a constant issue in the liminal space between the wine/mixology worlds–I don’t feel like I have any good answers there.
However, as someone who sells sparkling wine most days of the week and is constantly helping customers figure out how to parse the confusing world of champagne vs. non-champagne, I do feel like the “generalizing” issue is not as much of a problem as people would make it out to be (or, rather, it is an issue–but we’re focusing on the wrong pieces of the puzzle). The specific terroir of a true champagne, it seems to me, wouldn’t really be clearly identifiable in a cocktail that’s got bitters, gin, or any other spirit-like ingredient swimming around in it.
The more important thing, to my mind, would be that mixologists recognize that anything substituting for champagne in a recipe should also be made in the method champenoise-style (that is, with the secondary fermentation happening in bottle, rather than in a tank). It’s frustrating to me that folks get wound up over the price of champagne, and then go ahead and dump in a prosecco in its place. There are plenty of champenoise-style sparklers out there (cremant de limoux, franciacorta, and even most high-quality cavas are good, affordable examples) that would give you all of the yeasty, autolytic notes of a true champagne, which simply will never be present in a prosecco, or any other cheap sparkling wine made in the tank method. The flavors that develop from the wine being in closer contact with the yeasts during aging in-bottle, to my mind, are the components being lost when champagne is substituted for cheaper sparklers; so it’s really production method that matters more than place.