With the usual caveats on the danger of assumption...
Given the dire state of the Defence budget, one could assume that the option to reduce the Harrier Force and close Cottesmore earlier than planned (it did not have a future post Harrier anyway) was put forward as the 'least bad'.
So if we transfer to fantasy land where Facebook petitions have an effect, if Cottesmore was to be saved for a few more years, those savings would still have to be made so Defence would have took look at other - even less palatable - options. It would be good to see if the originators of he petitions have identified what they will cut in Cottesmore's place.