There's the economy. People are moving away from traditional Welsh speaking areas, and often in their place come English immigrants... what was it, just over 20% of the population was born in England? Something that has been ignored in all the media reports of an increase in foreign population...
When people don't find a reason to stay in the Welsh speaking communities, which are then also saturated with English speaking immigrants, the living language is going to suffer badly. The mostly English speaking areas have seen an increase in Welsh speakers, and that's probably a combination of speakers moving to these areas for work and because of Welsh medium schools, but like you say Miracle Day kids at these schools often hardly use the language otherwise. I know someone for example who is fluent and went to a Welsh school along with his brothers, but he and his brothers speak English to each other, as I read Carwyn Jones' kids do also.
The stats on nationality are interesting though. I think it was 60% or thereabouts that stated they were only Welsh, not British - and you have to remember that a good chunk of the whole aren't even Welsh anyway, so the real figure would be even higher. It's actually a similar story in England too. Seems Britishness is losing out... interesting with the Scottish referendum around the corner.