I don’t think there’ll ever be a time when everybody in Wales speaks Welsh (and honestly, I doubt there’ll ever be a time when Welsh is anything but a minority language). There are a few things that need doing in order to get more people to learn Welsh. First, we need more Welsh language entertainment, and it needs to be widely available. Not just television and radio, but films, music of various styles and video games.
I mean, you’ll never get the kids listening to male voice choirs (sorry, but it really isn’t going to happen), but hearing it in the styles that they listen to will make them more interested in learning what the words mean.
Also, according to Wikipedia, there are something like 12 films in the Welsh language in total. Even if we can’t make more films in the language, we at least need to get films dubbed in Welsh, or otherwise that’s another thing that people will only do in English.
For the television, there’s the usual, incredibly boring daytime television, there’s two soap operas, and there’s Y Gwyll. Again, Welsh needs more good television; the English language has tons of fantastic programs, from Breaking Bad and Game of Thrones for people who like their TV dark and brutal, to things like Buffy the Vampire Slayer and Doctor Who for those who like more light hearted fare. Again, there’s not enough money in Wales to fund all of this kind of stuff being filmed in Welsh, but some decent dubbing would make this something else people can do in the medium of Welsh.
And, again, there are video games. Localisation can be pretty expensive; I’m quite aware of that. That said, being able to play your favourite video games in Welsh would, again, give people something they can do in their preferred language. This would be cheaper in text heavy games, and more expensive in games with lots of voice acting, but it’s something else that people in Wales can typically only do in English.
These things won’t stop Welsh from being a minority language (like I said, I don’t believe anything ever will), but they’ll give people more opportunity to use the language - which certainly can’t hurt.