At the end of F&E, the Foundation controls about half the galaxy, and has the most powerful military. They could try to conquer the other half of the galaxy, but socioeconomic forces would have prevented them. Does the Foundation populace want to conquer the galaxy? Would they have the resources to hold half the galaxy against their will? I think there were lots of potential stories to tell. Part of the problem may have stemmed from Asimov having the Encyclopedia Galactic cite the war with Kalgan in the fourth century FE as the last significant
battle of the Interregnum. There was no need for that. He was just getting tired of the Foundation stories even back in the '40s.
The Robots-Empire-Foundation universe is a rich one, and a work-around to the "dead end" of F&E could've been found. They only went back to Daneel and Seldon because they're two of Asimov's most famous characters. But Daneel is better in small doses, IMO, and Seldon would not have time to develop psychohistory if he was out having adventures all the time. Giving Hari a secret life wasn't necessary to justify his in-universe contributions to galactic society. It's like giving Einstein a secret life as a spy on top of developing relativity.