Ds has had to write essays in English, German and Japanese (not at either Oxford or Cambridge as he wasn't interested in their courses and couldn't do Japanese with German).
He is thrilled to have done his last German essay in German, and is now concentrating on his dissertation which is, unsurprisingly, in English and is on a complex bit of German history. For his time in Germany he had to write all essays in German, and also took Japanese while he was there so he didn't get rusty, so for Japanese he was going from German to Japanese and back to German every day. That's quite a discipline when your first language is English. In Japan everything was taught in Japanese and written in kanji, not English (two hours of compulsory homework a night is set at Japanese universities) which would be a shock to many a UK student. At his uni, the contact hours for MFL are the highest after Medicine and Dentistry.
He started German in Y7, none of us speak German at home, and now speaks and writes it fluently. He started Japanese four years ago and now knows something in the region of 20,000 kanji (considered to be the number required to read a newspaper such as the Japanese equivalent of The Times), and he had to do essays in Japanese when in Japan. He speaks it to an accomplished conversational level, fluency being considered hard to achieve in Japanese without far longer immersion. There are roughly 50,000 kanji plus hirigana and katakana so it's a lot to cover. He does not speak what would be considered 'formal' Japanese (which is what they speak in the recent Shogun adaptation) as UK universities mostly don't teach that because it's starting to die out in a slowly less formalised Japanese society. He reads Japanese books, and watches anime so actually uses and hears Japanese more often than he does German, but communicates with the friends he made in each country in both.
I can't see any of that reducing written fluency, and I also can't see where four years of that fails in a calculation of academic rigour. The changes that have come about with the year abroad, where many students now attend university in their language countries, rather than working, mean they probably do more essays in their languages, now, than they used to.
It's a depressing line of thought that some follow: "the English teach languages shockingly badly, no one progresses above A level standard during their time at university, it's a total waste of time as they will earn no money and be in lots of debt". The English way - shout loudly, people will understand, we don't need to speak their lingo. Given the levels of fluency amongst his friends who have taken various combinations of languages, I'd say the fluency/A level standard argument is totally spurious.
He may, or may not, use his German in the future, depends if he does go in to teaching. He will undoubtedly use his Japanese as he is back there in the not too distant future, post graduation, and will continue to read and watch Japanese output as well.
Like others on this thread with children who have done MFL, I'm very proud of how he has learned and developed over the last four years, in both languages and resilience (Germany was a bureaucratic nightmare and dealing with that in German was... character building...). I don't care one jot if he isn't as good at Japanese or German when he gets to the age of 90, as someone's granny who went to Cambridge and still read her Proust aged 106. I won't be around to care after all.