The style of offense Colt ran isn't necessarily traditional. It was a spread and spread offenses are relatively modern. Also Harsin's offense was successful, there's no denying that. So in one sense I'm disappointed to see him go because I actually liked most of his play calling and the style of offense he runs. With a better QB his pffense would be even better. But he may have rubbed some players the wrong, way which may shed light on the rumor of why M. Brown thought about transferring or why Juco QB's have been deciding to look else where.
If Harsin does leave I fully expect Major to take over as OC. I don't see Mack bringing in anyone to mix things up even more on offense after making positive steps this past season.
If Applewhite coaches with the same personality he played with then I wouldn't doubt players respect him and enjoy him as a coach/coordinator. Which could be a positive in recruiting this off-season. We don't know much about Applewhite as a play-caller. But I do know he was a smart QB and has done a decent job in his short opportunities to call the plays. I assume if Applewhite becomes the lone OC that Mack would also have more say in the play calling, since he is an offensive minded guy himself. I don't think Mack would have a ton of say in it, unless Applewhite began calling bad games. But i do think he would mentor Applewhite in his first big gig.
With this all said, if Harsin does leave then I'd be all for seeing what Applewhite can do.