For me, Gretel is Big Brother Australia. She was, and to an extent still is, the face of the show in this country.
However, as much I absolutely adore Gretel in so many ways, there's a number of reasons why she just wouldn't be right for the show anymore. First of all, she has moved on, plain and simple. But in addition to that, the host of a live show needs somebody who is current and also in-touch with today's viewers. Unfortunately Gretel doesn't fill that quota anymore and, apart from her stints on Mornings (does she still do that?) she has pretty much dropped off the radar nowadays.
To quote myself from a recent post I made in another thread, the problem with Gretel and her current relationship with Big Brother (if her recent interview in 2012 on ABC's One Plus One show was anything to go by) she seems to now hold some sort of disaffection towards it and the time she spent hosting it. I know she certainly loved it for several years during its prime (and she was bloody fan-effing-tastic at her job, probably the best live TV host this country has ever had in my opinion) and she dealt with hell of a lot during her time on BB (more than most other hosts ever would, or even could) but she definitely lost touch with the show towards the end.
And in all honesty, I don't blame her for the distaste, considering the way she was (in my view) unceremoniously dismissed in between seasons. Regardless of whether it was a mutual decision or not, personally I will never be content with the fact that she never received the on-air send-off/farewell that she absolutely deserved - and that phone call interview at the BB08 Finale doesn't count as a send-off either.
But yeah, unfortunately Gretel wouldn't suit the show today, nor any show to be completely honest. I'm sure she's quite happy living away from the spotlight too after being a very prominent figure on our screens for a long time. Best wishes to her, I say.