It'll obviously be McGregor, Disney does a lot of stupid things but a) people associate Obi-Wan with McGregor and there is no lore/age reason to not cast him b) McGregor is actually a really good actor.
What worries me is where they're going with these one off character movies. I am not inspired at all by what I've heard from the Han Solo movie, and I'm still really worried that they're going to go with the "Boba Fett is an assumed identity not a person" even though Temura Morrison was retconned into the OT to be the voice of Boba...they my as well hire Tommy Wisseau to direct if they go with that horrid idea.