I thought I'd read that children who are multilingual, tend to be more intelligent.....or have I just made that up, who knows?.
I've put my daughter into a welsh first language school as I believe that it will give my daughter a far better chance of a decent job, once she finishes her education, should she want to find a job locally.....and that the Welsh language should be kept alive, of course!
The problem can be that a focus on Welsh can sometimes lead to the child having a substandard grasp of English (compared to those children who have English as their first language), which can lead to many difficulties later in life if the child wishes to go to study/work in England/Scotland/America/Australia etc etc. There just aren't many well paid jobs in Wales, sadly. Just being pragmatic when I say this.
I think the problem is with the education system in Wales, in that it's just not up to the job of providing a comprehensive education in both English and Welsh.
If you are saying that education through the medium of English, with Welsh being taught as any other subject is wrong then I disagree with that completely.
My education was done in English but we had Welsh, albeit compulsory being taught in the same way as any other subjects.
It didn't stop me from obtaining an O level in English grammar but I'm still involved with the Welsh language whereas I've had no need since leaving school to use any Latin or French that I may have learnt there.