Well how do we know that Comcast would sell the Marvel parts to Disney and not to someone else? They might try to sell off other assets and hold onto Marvel just for the sake of it.
I think it's highly unlikely they'd sell them to someone else. First off, the rights probably aren't transferable. Since Comcast is buying Fox, the rights will come with the studio, but selling to a third party is a whole different thing.
They're also not worth much to anyone else since they can't integrate them with the MCU and stand-alone FF films have a history of failing.
Marvel wants them. I have no doubt about that. Dr. Doom, Silver Surfer, Galactus and Kang have a big value/importance to the MCU just by themselves.
But there is the possibility Comcast would set a price Marvel wasn't willing to pay (I think that was the problem with Fox).
Some around here are convinced Comcast would make their own films. I'm less convinced of that because I don't think it makes financial sense. After the damage Fox has done, FF films specifically would have to be integrated with the MCU to have any real value. Comcast just repeating what Fox did is likely to produce similar results, and that's not a big money making proposition (particularly when they don't have merchandising).
There's also the issue of a tight timeline here. An FF film needs to be in active production by 2022. That means script, director, pre-production etc., should be starting by about 2020 to do things right. If everything gets scuttled and they have to start over, I wouldn't expect Comcast to close until mid to late 2019 at the earliest, so that doesn't give Comcast a lot of time to really think about what they want to do before being forced to begin pre-production of an FF film.
With all that in mind, I'd expect Comcast to either partner with Marvel or sell to Marvel and the latter would be the neater, cleaner and probably preferable to both parties option.
