Asimov being Asimov, the human consequences of the decline and fall of the galactic empire happens mostly of screen.
How exactly Trantor in a couple of hundred years went from a bustling planetary city to a planet where the last survivors scratch out a living from farming the former imperial grounds, is better left unexplored. If you are living in that world you are much more likely to be among the masses were stuff happens that will eventually be noted by Foundation scholars as "population decline", than being a Foundation scholar.