Disney never openly fired Johnny Depp in his role as Captain Jack Sparrow, unlike, say, James Gunn being fired from Guardians of the Galaxy Vol. 3 after past tweets. Disney reinstated Gunn after several months of fan outcry, but any Pirates decision may also be based in economics. Depp is an expensive star and the most recent film wasn't as well received as the studio hoped. They may save money by going with a reboot, although that would be a gamble too.