The decision was all about the number of frames funded to maintain the "required" FE@R and this is where it all gets silly and also where it is deadly serious.
FE@R is essentially a funding assumption which drives a lot of budget line items under the DLODS. There is a tendency among those in MB and elsewhere to believe that FE@R is somehow more than this, something that is set in stone, which it was never intended to be - it has grown out of the old Defence Planning Assumptions - and can be changed.
However, changing it requires funding and that is where it gets deadly serious, if you end up driven to the lowest sustainable FE@R value (which JFH had been). I don't know if anyone actually looked at the relative FE@R provision between the GR7/9 and GR4 forces and whether they could be rebalanced for an affordable solution across the capability requirements. However, once the argument became about losing a "fleet", then the fleet with the smallest FE@R was always in trouble. Anything that got in the way of saving money was going to be doomed under SDSR.
As far as the USMC are concerned, good luck to them. The fact that there are no AV8 in AMARC suggests that there are some very tired frames out there which will probably go to AMARC as our "low-time" frames go through a regeneration phase at D-M. If they think they're really going to replace some FA18D in the force structure, they may have a nasty surprise coming.....