Ugh - Doc Brown preaching a climate change storyline. Thank god they're saying no.
Greta could play Biff's granddaughter - she'd actually be a great villain, maybe for a Bond movie.
They're taking the high ground but you do have to wonder if MJF didn't have Parkinson's if they might be saying something different though.
They would realize that so long as Fox is alive there's not a fan-accepted way to do a new BTTF (given he has acted a fair amount with Parkinsons, incorporating his illness into the story,) yet using him in a new movie (main character's dad or whatever) would be really tricky to do tone-wise.
Personally, I think it would be an amazing thing to see onscreen, but a studio would freak out about it. And it's possible that MJF himself wouldn't be okay with it. It's an interesting discussion.