While I generally agree with you , I think you are making a mistake in saying Coogler has to rewrite the entire script. They can very easily keep the script and ask if you can make another character the main focus of the same script. And yes while that is some work it isn't blowing up everything up. And then considering how other characters had large part in the other film and everyone is back, that lets you know that is another part of the film's script that doesn't need to be blown up.
Now the most telling thing to me is how they have said they aren't going to use CGI to include the late star in the film that quickly narrows down what they are going do and when you consider that they are filming so soon in July. Your point seem right to the recast but Coogler has a co writer and they have access to other writers which turns a lion task into reworking something much easier. Don't over look Marvel saying to Ryan "we like your script can you put blank character as the lead" and Ryan going let me think about and get back to you. And maybe a few days later Ryan gets back to them "we obviously have to make some changes but I can think adjust this character to be the lead".