To the extent there is any current need for Amtrak service to GCT, it really only has to do with the fact that Grand Central is more convenient to a lot of the big midtown office buildings. I would guess that if you polled riders on the first two trains from Albany to New York in the morning, you'd find a significant number, perhaps even a majority, who'd rather arrive at Grand Central than Penn if given the choice, especially if they could walk to their destination from GCT and would otherwise have to use transit or cab from Penn.
But even for those people, Penn is not that inconvenient. It's not off in Siberia; it's still in Midtown and has lots of transit connections. And in fact, it's close enough that very few people seem to bother with changing to Metro North to go to Grand Central. Changing at Harmon or Yonkers introduces uncertainty and the risk, particularly outside of rush hour, of a blown connection and a resulting delay of 30 minutes or more. Better to stay on to Penn and rely on the subway, which runs much more frequently, to get to one's final destination.