A couple of general points. I've followed railroad news for a long time, but I'm not a professional: somebody PLEASE post and correct me if what I say sounds wrong!
(1) Electric freight on the New Haven was probably a lost cause. The tunnels through Penn Station in NYC are not suitable for freight operation (clearances, steep grades), as I understand it, so the New Haven's electrification would be an isolated 80-mile line: not economically worth maintaining a separate locomotive fleet for and changing engines when freights entered or left the district. (Why had it been worthwhile earlier? I think the economics were different! Time and labor costs of engine changes are more significant now than they were when NYNHHRR ran electric freights. And, of course, the New Haven's last electric freight locomotives -- the EF-4 a.k.a. E-33 -- were acquired second-hand at what was undoubtedly a bargain price: they might not have re-instituted electric freight operations in the early 1960s if they had had to pay new-locomotive prices.)
(2) I think that the Conrail management when they started weren't sure what would be the best strategy: I doubt they were committed from the outset to an all-diesel operation, and if things had worked out a bit differently (Amtrak offering lower prices for using their tracks, for example) they might have kept electric freight operations. In 1976 there were people who saw a bright possible future for freight electrification:
-------EMD built two demonstrator freight electrics, and GE rebuilt one E-44 to a 6000 hp rating: I doubt the two big locomotive builders would have invested in these projects if Conrail had let them know right from the start that they were going to phase out electric freight a.s.a.p. (Mind you, 1976 was a time of widespread awareness of energy issues -- the Arab oil boycott and 55 mph speed limits were fresh in people's minds -- so the idea of new electrifications in North America didn't seem totally ridiculous! So maybe the locomotive builders weren't JUST hoping for Conrail orders!)
-------There was some talk in the trade press (I recall, vaguely, and article in "Railway Age," but don't recall details) about extending the former PRR electrification to Pittsburgh in the period around the formation of Conrail. Remember that Conrail received a lot of government money to help it get on its feet: the idea that the Feds might lend, or underwrite a private loan, for an ambitious rail infrastructure program wasn't totally outlandish.