Aren't these just growing pains?

You know, I have some vague understanding of the problems that a decreasing birthrate/aging population can cause, but really... Whenever I hear people lamenting or even warning about the lowered birth rate in Japan, I think to myself: Isn't this a good thing? Japan's crowded enough as it is, as is much of the world, so in my mind issues like difficulties in supporting the aged are really just unfortunate growing pains on the way to something better.
Is there something I'm missing?

P.S. From the J-List newsletter: "Hopefully, they'll come up with a way to keep their race from dying out between now and the year 3000, when the number of Japanese in the world is projected to be...27." Umm... Please tell me he's kidding. I'd bet you 1,000,000,000,000 yen that, barring some sort of disaster of the sci-fi sort, that this birth rate trend won't continue until the population of Japan hits even 1,000,000.