Perhaps if James Gunn were fired prior to completing the script, things might be different and Bradley Cooper could have been brought on to write and direct, but at the moment, it seems like Disney still intends on using James Gunn’s script for Guardians of the Galaxy Vol. 3.
Bradley Cooper has also been supportive of James Gunn after his firing, along with the rest of the Guardians cast, so it seems unlikely that he would even want the job, whether he was writing or not.
You have to think Disney wants that too considering the success of the first two Guardians films.
That’s certainly what fans want, desiring to see James Gunn’s vision, style and humor preserved as much as possible even though he isn’t directing the film.