I also think it's entirely possible for consumer tastes to change, regardless of if one keeps outputting the same thing, as the individuals who comprise the market segment shift. They grow up in different environments, economies, etc. The 16 year old in 1990 eagerly awaiting the next Super Mario, is 40 now. The 16 year old boy in 2000 eagerly awaiting the next Final Fantasy, is 30 now. The 16 year old now is eagerly awaiting the next Call of Duty. In 20 years time that 16 year old may want something entirely different, on the assumption we have consoles at all. That isn't to say that fanbases necessarily vanish, but I think they can diminish, or move as you note, and that movement can be due to circumstances outside of control (for instance you note the shift to handhelds in Japan).
As aforementioned, I think the primary market of the PS2 was playing different games/franchises/genres, but I don't think the people (demographically) that comprised that market is hugely different to that which drove the PS3 and 360 and that which is driving the XBO/PS4 now and going forward. I think there was certainly a market segment within the PS2's installed base particularly conducive to the types of games the Wii brought, with things like DDR and SingStar, and whether that's still present is a question. But I don't think that was the primary driver of it's success and it came later in the cycle, so it's far too early to me to determine whether they return. I could certainly be wrong.