but what do MFL teachers use their languages for, if not teaching?
It's an interesting view point. One, I think many do hold (though I'm assuming Quair what you meant was what do MFL experts do with their skills other than teach?)
DD is doing 3 MFLs, hoping to take 2 and English as her other possible subject choices higher up. She says she keeps purposely vague now when asked by parents of her friends because they naturally default to asking if she wants to become a teacher. And treat STEM subjects more seriously. DD holds teaching in the highest regard, because of the wider challenges of the role beyond just subject matter, and doesn't think she'd have the skills (yet). Her aspirations are also not with pure linguistics either, but using MFL and English as alternative routes into business/IR/political science type roles vs the traditional pure economics or business related degrees. So the undergraduate courses that include history, culture of the country/regions are of interest, as well as developing her fluency.
There are also the concerns that native speakers are at better advantage on courses, contributing to the decline in take up in non native students. Again, this may be true, but she schools with many students who you'd never know immediately what their parentage is. And whilst their spoken word may be fluent, with obvious benefits of more enriched vocab, their structural/grammatical knowledge is often no better than non native. It's another non issue for them, more so for parents looking for the disadvantages their non-native kids have and what needs to change to level things out at HE level, like declaration of backgrounds etc. Over-thinking it is her view, especially in a world that gets more diverse!