I am English, Welsh learner, living in Ceredigion.
My husband speaks Welsh at home, as do his family who we have a lot of contact with. Our son goes to a Welsh speaking school, his friends speak Welsh, he watches Welsh TV (as well as English) and reads Welsh language books etc etc. I speak Welsh with him too, but it's my 4th language so I find it difficult to speak all the time at the moment!
However, his English is better than his Welsh and he speaks mostly in English (which is fine).
My husband and SIL are both scientists of different kinds and have not struggled at all with technical/academic language. Their grammar in both languages is excellent.
Although we might not notice it day to day in Wales, English is everywhere; the kids I know who only speak Welsh all day still have no problems learning English because of this.
Also, there seems to be an idea that one maternal language works against the other, which is just not true. They don't cancel each other out, they help each other. It's not the same as a second language.
All in the name of welsh nationalism ... Yes. I'd be genuinely interested to know more about education suffering when it's completely through Welsh. Do you have a link?