Reading about and the interviews Stan Lee and Spider-Man writer afterwards Gerry Conway had about the past paints not a great picture to Gerry towards Gwen in fairness vs MJ.
When googling 'Spider-Man Conway" in google amoung the first reports that show up is called "Gerry Conway Blasts Gwen Stacy, Criticizes Stan Lee in New Book on Marvel".
From that report:
"In a just-released excerpt from the upcoming book Marvel Comics: The Untold Story, legendary Amazing Spider-Man writer Gerry Conway had the unique ability to kick a character that he had already killed while she was down when discussing the decision to kill Peter Parker's girlfriend, Gwen Stacy, rather than his elderly Aunt May.
“She was a nonentity, a pretty face. She brought nothing to the mix. It made no sense to me that Peter Parker would end up with a babe like that who had no problems," Conway is quoted as saying in the book.
"Only a damaged person would end up with a damaged guy like Peter Parker. And Gwen Stacy was perfect! It was basically Stan fulfilling Stan’s own fantasy. Stan married a woman who was pretty much a babe," Conway continued. "Joan Lee was a very attractive blond who was obviously Stan’s ideal female. And I think Gwen was simply Stan replicating his wife, just like Sue Storm was a replication of his wife. And that’s where his blind spot was. The amazing thing was that he created a character like Mary Jane Watson, who was probably the most interesting female character in comics, and he never used her to the extent that he could have. Instead of Peter Parker’s girlfriend, he made her Peter Parker’s best friend’s girlfriend. Which is so wrong, and so stupid, and such a waste. So killing Gwen was a totally logical if not inevitable choice.”
So he saw MJ as 'Broken/damaged' and only 'broken people can be together' viewpoint. Reading the past issues I can't see how MJ was broken (note only taking from the series began to #121 ONLY as a source like a reader of that time period and not what writers game as developments or recons as in their power (and Gwen could of had in turn). Also I wasn't a fan of MJ that much till and after #122 kick started her path in developments and her developments so that's good for her and that MJ she developed into was a MJ I became a fan of (though 616 MJ has had a rollercoaster in my view of developments/positives but also negatives making her fall in the midway of favorite MJ versions right now but always a possibility to rise by writers). The past first 616 MJ presented to me in those issues as a 'party girl' only and 'selfish' (and didn't know Peter was Spider-man by seeing him jump out his window as Spider-man as that came much later in a recon) before 121 then 122 with Gwen's death being the writer's catalyst/started the movement to a self-less hero developed MJ that I'm a fan of. If that's what he meant as 'broken' being her 'party girl' and 'Me Me Me' and he had ideas to alter that I would be happy for a writer to ideas that allows character growth and desire such. But I don't feel that was the goal really.
In other interviews he has stated that he read stories (a influencing factor in his writing) where in those tales characters unexpectedly died. He felt it kept it interesting. Then Marvel saw declining sales and thus wanted a 'death' issue to push numbers up (same old story as today). Aunt May was going to be killed (in doing so we might of also lost so good developments and versions of Aunt May presented by other writers and influencing factors like the movie Spider-Man: Enter the Spider-verse Aunt May that I loved) but Conway pushed for Gwen to die.
He wanted the girlfriend/'MJ competition' out so he could pair up Peter and MJ. If he would of been denied he stated that he would of had both Gwen and peter break up and Gwen leaving for MJ and Peter to get together. Instead of seeing a weakness and then form questions to why then answers in character developments and plots for Gwen to become better he just killed her and possibilities in character developments for other further writers who might of had ideas. So Gwen died after her father died before so MJ and Peter can hook up and push sale numbers up. I've seen this before with fanfic writers in shipping wars and 'ron the death eaters' tales. Writers are human and have personal views and desires and Conway desired MJ and Peter together and had ideas for MJ mostly and the two coming together and Gwen to him was a pest to kill then develop.
Overall as Stan Lee said "he wrote the stories he wanted to write" and the same applies to all writers. We might not agree with what they want however or share viewpoints, effect by different cultures, or see different directions or possibilities.