Weekend mail call:Isaac from Salinas, Calif., wants to know if I think the 49ers could try to replace Jim Harbaugh with Jon Gruden or Bill Cowher in 2015.Bill Williamson: I really doubt it. If the Harbaugh situation blows up after this season, I would think the 49ers would first look at an existing assistant such as offensive coordinator Greg Roman, defensive coordinator Vic Fangio or defensive line coach Jim Tomsula. I wouldn't think a big-name, out-of-work coach is where the 49ers would first look if somehow Harbaugh is not in the picture.