I think some of it is the fanbase overestimating how thematically important the Goblins actually are. I mean, think about it: for being such an allegedly irreplaceable archnemesis, Norman Osborne spent several decades just plain flat out dead. Until his modern revival ( which has largely been execrable ), Norman Osborne's primary continuing importance was "motivational factor for Harry's life and periodic villainy". Even Gwen Stacy's death, was influential in that Gwen Stacy died, not in who did the killing. You otherwise had villains like the Hobgoblin, who stole some of the image and gadgets but otherwise had their own completely different story and themes, that might as well not even be connected to Norman Osborne.
Which is my meandering way of saying that they could simply never introduce the Osborne's at *all*, and Spider-man would be just fine. He's got plenty of villains with at least as many *good* story arcs to adapt, and with at least as good or better thematic resonance.