Originally Posted by
Mister Mets
JMS's plan was to have Mephisto change history so that Harry Osborn never got addicted to drugs, which would have prevented Gwen Stacy from ever bonking Norman Osborn. And also dying.
If your objection to Sins Past is what it said about Gwen's character, JMS's retcon changed nothing. It wouldn't change what she's capable of under the wrong circumstances.
Quesada had plenty of other reasons to veto JMS's plan, given the effects of Gwen's death on the Marvel Universe.