How many sectors are the F100s doing a day? I had always understood that a lot of the FIFO flying was low sectors, with a lot of downtime for each frame - meaning that low capital cost of acquisition was a major factor.
Easier to be profitable when the depreciation/lease cost of the plane that spends most of its time on the ground is almost zero. Harder to be profitable, if the plane is burning money sitting on the tarmac.
If Bain really want to simplify to just 737s, I reckon there'd be a good chance they would sell VARA and contract the flying.