The rights to Star Wars (77) aren’t worth all that much, honestly, considering they already have what matters most. Even the other Marvel stuff probably isn’t worth more than a couple billion at most (very rough guess). The main goal I believe in terms of back catalog is for more content on their streaming service.
But of course it’s not just buying their back catalog, what’d probably happen would be that Fox would keep chugging along more or less like before, just as a Disney subsidiary (along with the other Fox subsidiaries in the deal, depending on what Disney wants to do with them). Same thing with Comcast.