I'm an MFL teacher so feel free to ignore anything I say as I'm very obviously biased!
a) 'everyone' does not speak English. There is huge evidence and therefore concern that Britain misses out on billions in business annually as a result of our inability to do business abroad. Even with a Masters degree under your belt and years and years of work experience, you absolutely never know when having that GCSE in French might just give you an edge. Engineer looking for a job - company just won a contract in French speaking Africa. Who gets the job - the one with the GCSE in French or the one without it? Who gets to ask for more money in that situation - the one who can begin to take on additional responsibility because of their language skills or the one who passes the phone to their colleague when a foreign accent is heard?
b) it is true that some courses at some universities require an MFL GCSE. It is also true that some of the more popular courses where demand is high and the course admissions tutors can pick and choose , 'good' candidates will be selected on the basis of their 'all roundness' and 'quality of education'. If there is serious concern about university with/without a GCSE in one of the MFLs, talking with the child concerned about how they see their future, having a look at course outlines online and, if necessary calling/e-mailing a few admissions tutors to get an idea of whether or not it may affect future options is the best way to go.
c) I would suggest any student who is otherwise pulling A/B predictions and has an interest in business, politics, history, art, drama, English, engineering, that kind of thing, would do well to try and get a language under their belt. Anyone likely to do some kind of 'year in industry' as part of their course will likely benefit as well. It is also worth younger students looking at university courses now - they may have an idea they want to study 'business' but if they start to look at courses, they may suddenly see a year in a foreign country gaining business experience as something they are really interested in. This kind of option isn't uncommon - and the doors can be closed to students who don't have a GCSE in languages. I, for example, studied Spanish at university from scratch as part of my course - but I wouldnt' have got on that course without my 'O' Level (I'm old!) in French.
d) ultimately MFL is viewed as an 'academic' rather than 'soft' or 'easy' option (hence inclusion in the EBacc) which obviously needs consideration, depending on how you see yourself and your future.
Hope that helps, rather than confuses further!