I’d disagree with the premise of this post, I think there are a ton of newer Pokemon from gens 4+ that are iconic and will continue to be popular. This thread was great for talking about it:
As I noted in that thread, there’s a linear relationship between age of a Pokemon and how “iconic” people think it is, not necessarily because of anything objective, but because of nostalgia and time passing.
Lots of newer Pokemon are already hugely popular as others have pointed out.