How do you mean 'failed'? They made a choice to take a different path, the outcome of this will be interesting, as almost everyone else is taking the immigration route to fill the demographic hole, but I'm not convinced it's going to work out as intended.
I'm not convinced on immigration either way.
Immigration of suitable people (people who add value rather than drag on the economy) to keep the population stable seems like a fair idea.
Adding people to 'grow' the economy is dubious in my book.
Japan did neither - I'm not sure if they made a concious decision knowing the population would fall or if they really took their eye off the ball.