So persuading Marlon Brando to play Jor-El was an expensive gamble that initially paid off. But repeating this formula with other actors (none close to Brando's near-godlike mystique) progressively doomed the entire franchise.
Nope. They got rid of Donner. That's what doomed the franchise. All the other sins follow from that cardinal sin.
Everyone else involved -- Richard Lester (light comedy background), the Neumans (who became involved through Superman: The Musical), David Odell (the Muppets) and the Salkinds --
wanted to produce campy Superman and Supergirl movies. While it was abundantly clear that the cinematic Superman/girl worked best "straight", with only sporadic bits of mild camp and corniness as a lightening contrast, the Salkinds wanted what they thought would be laughs and levity. They acquired what they thought were the right pieces, got what they felt they wanted, and it sucked.