I'd like you to know that you've built a coherent, reasonable point on the subject of taking Maths course. You said "I think everyone should take that course. Would it better, though? I don't know". It's okay to provide your opinion and whatever arguments you hold that support it, and it's okay to admit that yours is an opinion, not a proven fact.

I think it's a case of professional myopia. As a linguist and a language person in general, I'd suggest everyone to take at least one foreign language class, because it's better for ya. I can clearly see the benefits, but mostly in people already inclined to enjoy the advantage of language-learning. You, as a person of strong inclination for STEM subjects, broadly speaking, would see similar benefits in people studying what you consider basic Maths (which some of us dread because of how rigorous the studies are).

We aren't wrong in suggesting that studying this or that subject could be useful (especially as basic and everyday as mathematics and language, even one's own). I'm not to speak for you, but personally, I've spent too much time around people already inclined towards and enjoying learning foreign languages to see clearly what the situation is like outside this fairly narrow circle.

I would suggest doing some research on the matter. Perhaps make a paper on the matter of benefits of learning mathematics among humanities-inclined students. I, for one, would love to read that, but it might also benefit the inter-field communication where fields become highly specialized without developing auxiliary skills that don't pertain to their subjects unless life suddenly prompts them to it.

Lastly, as a writer and an aspiring teacher, I'd say the remarks you've gotten on your essays are shallow and incompetent. What they say mean nothing, and so does the teacher signing them. I can empathize with the teachers' workload - if managing your workload is testing, imagine managing that of your whole class - but I can't empathize with bad work. Reading a 5k-word essay is hard, man, especially when it's far from the only one. That is to say: it's not that maths are necessarily so much more understandable. I'm not going to deny the very nature of the matter, which is very precise and tracable, but I'm here to tell you that yours is merely a bad example, not the baseline for the whole field.