If Virgin currently have 75 B737s and Bain are looking at a fleet of 60 - 70 aircraft then worst case is a 20% reduction ie 172 pilots. Normal retirement and loss of medical will come out of that number, a few may decide to take early retirement or change careers. Without a generous VR package on offer, and very poor job prospects in the short to medium term, most will likely hold on and hope for the best.
Bain’s view of Virgin could be a rough indication of possible QF restructuring on the domestic side, basically 10 - 20% less. International is a different matter as Virgin can easily do without it however it’s a QF core business which needs to resume when possible however it’s the most difficult to predict. All that can be said for certain is that the network and frequencies will be reduced and smaller aircraft used. B737s will replace wide bodies where possible, which may take up some slack on the domestic front.