I think they know a Melissa spin-off would bomb, hence why she's being kept on the show.
It's clear that Bravo (and Andy especially) like Melissa and Joe. For all their faults, they seem like they're nice to work with and aren't a living hell for producers.
Everyone and their grandmother knows Teresa is a star, so it's interesting to see they might go ahead without her. I imagine Luis is a massive headache for them and is probably what's causing them to be hesitant. He's unpredictable and a big part of why the show became unbearable for the cast. It's much easier to keep them in the fold and give them their own show, which I think will be fairly successful.
It's sad to think we'll no longer have Jen and Marge, they're essentially the same person in terms of how they operate but it's clear, as mentioned above, just how affiliated they are with bloggers/leaks and that can't go on as it contributes to the toxicity that ended everything in the first place.
To me, it's obvious that Dolores wasn't going anywhere. Yes, she straddles the fence but she's a big fan favourite and keeps the group grounded. She had to come back. I just hope Jen Fessler gets a shot as well!