I doubt Munn will go anywhere. She dosen't have enough exposure yet to really do anything. Give it another year or 2 and a few more movies. She may be gone from hosting for awhile but she will be back. Webb has her own thing the next logical replecment is either Haislip or Underwood.
Webb's own thing is X-Play which requires her to be in studio for ten minutes a show. Well I know it's actually more than that, but it's not a laborious work day...she's not a writer. For some reason, I imagine it is her decision that she's not used as a fill-in more often. She does a good job when she actually does co-host.
Anyway, Munn has said that she'd like to do the show for years to come, but if she actually landed a show and/or gets any movie roles that require more than a week of shooting, there's no way she could do both.