Didn't read malstrom's article, but anyone who thought Wii Fit would suddenly signify the replacement of the core demographic may have placed a little too much stock in the product's importance.
Best case scenario: more fitness based titles are released, marketed directly at the new-ish demographic of Wii owners. And that means more copies of Wii Fit sold, unless Nintendo starts selling Balance Boards separately for $80 or less.
Most gamers aren't particularly obsessed with fitness in general. I'd venture to say that for most core gamers who were eagerly anticipating Wii Fit, the fitness game concept is a flash in the pan. They're not going to suddenly stop gaming and start spending all those hours exercising, be it video based or gym based instead.