I think you're misunderstood the point of my post.
There are 3 categories of DC:
1 - Who would do MFL no matter what.
2 - Who would like to do MFL, but are deciding between it and competing options.
3 - Who don't know what to do, and for whom MFL is an option.
The difference between 2 and 3 is that for 2, they are choosing between MFL and another passion. For 3, they don't have any particular passion, but MFL can be an option if it is ab-initio. Unlike something like say, mathematics where you need to have done it at A-levels.
The demand for MFL from 2 and 3 is dropping. Why? Because there are other options for learning a language and so, if your liking for it was lukewarm, you would do something that you could not pick up later on life, or on the side. I would advise my DC to do exactly this. If they REALLY wanted to do MFL of course, go ahead, but it's the ones on the fence that are contributing to the drop. And here's why.
A few decades ago, when there were few avenues to learn another language doing it academically at A-levels, and then going on to uni to do it was the only way.
These days, if you 'love languages' there are so many other options. Therefore, people choose other things for their academic pursuits and learnt languages on the side. Biology is a bit of a silly example because you can't really do it on your own. A more appropriate example would be music. Many people have music lessons but few choose to pursue Music degrees compared to the population who studied it.
I'm not sure what experience you have with MFL but my point wasn't that people learn it from a 'TV show' - if you read my sentence properly, it was stated as a medium that ignites interest, alongside For example, people who are interested in anime and K-pop start with TV shows, it ignites their interest and they go for formal lessons. Language qualifications in Europe for example have to follow the CEFR framework and these include an awareness of an discussion in culture, politics, art, all of the things you mentioned. Anybody can learn and do the exams. C2 level, the highest confirms proficiency at the highest levels of the language including academic proficiency. Plenty of other people around the world learn this way, because other cultures value languages. Because they want to go to university, because of TV shows as I said earlier, many reasons. They don't moan, like you 'omg we don't have A-levels and university degrees in this, so we cannot learn any other languages'. And many of these are not the languages they speak daily. If like you said 'they don't need to other than speaking English' well, that's their inwardness to blame, isn't it?
Your final point also supports the crux of my argument. Exchange programs are expensive, and other students are not more deserving than MFL ones. Therefore, if everyone can go abroad. What is the incentive for them to choose MFL for a degree, when they can do something else and still keep their language learning up? Even in my own university many did exchanges, specialist for their own degree subject, aided by their CEFR qualifications done alongside their degree. It opened more doors for them than a 'plain MFL' and many of them did humanities subjects.
Of course, if MFL is the only way people can go abroad interest will re-ignite but, in your own words that's not an outcome that's desirable either because, well, erm everyone should have the chance to go abroad.
Which, again, university is not the only way, I have been on funded language exchanges through things like the Alliance Francaise. Coming to study in the U.K is by the by but many of my peers have gotten jobs, done secondments, etc without it being provided by the 'university'. Yes, it's a shame Brexit has closed so many doors, but there are other ways, as people in other countries have to do. And I'm not even from a privileged background. If you're talking about funding so many Brits take a gap year, there are working holidays etc you can get a visa for on a British passport that people from 'developing' countries like me won't even get the chance to sniff at. You're still quite privileged compared to people in many other countries.