"Speak good English and the World is your oyster, speak good Welsh and you get a few very pretty hills."
A child whose first language is English won't stop speaking it just because they are in Welsh medium education, though.
Smiling - my dd is in yr 6 at a Welsh medium primary. For background, in her year of 16, I think that 2 children have parents who are fluent Welsh speakers, and none speak Welsh at home AFAIK - which is pretty typical of the school as a whole.
I would say that there are advantages and disadvantages. DD has some specific dyslexic/dyspraxic type issues, and I do think that working entirely in Welsh for KS1 didn't help with these (in particular because she was reading fluently in English by the time they started using it in school in yr 3, and so got no English phonics at all). I think if your dc have any ALNs then working in a second language - and a language which realistically in many parts of Wales they are unlikely to use outside of the school setting - adds an additional layer of difficulty.
Moving on into secondary, there are other issues - in theory dc may be fluent, but if they have only spoken the language in school then they won't have the range of vocabulary that you would expect. And it is hard to expand that vocabulary since the resources easily available in Welsh are pretty limited (even at KS2 try helping your dc find the vocab for a piece of homework talking about how chamelaeons change colour). Too often I would say dd writes a piece of work to the words she knows, rather than to her true ability.
The overwhelming obvious advantage of course is if they stay in Wales then it will help with employment. So often two people I know go for an interview, and the Welsh speaker gets the job, of course this is going to happen if there are two equally qualified candidates, why would you not choose the one who can speak to your customers in their preferred language.
We are now at the 'welsh stream or english stream' question for 2ndary. DD isn't going to the welsh medium secondary despite its stellar results - we don't think it is right for her partly because of the school, but largely because of the Welsh (though also tbh I think a lot of the very traditional attitudes are closely linked to Welsh culture in this area). She has the option of streams in the other school, ideally she would do just a couple of subjects in Welsh, which used to be allowed, but seems not to be the case now . . . a big dilemma for us.