Yes the only way to have more than two missions at a time going is to build FOB, that seems to be the only real benefit, which is not that important. The missions are done like every 20 minutes.
I don't disagree with you that they don't take that long. But if I'm getting this right... someone who pays for these FOBs can do multiple missions at the same time while regular players have a limit of two missions.
That means people who pay real money get more GMP, resources, blueprints and other stuff.
It would contradict everything they've said. It means the microtransactions do have an effect on singleplayer gameplay.
It's pretty shady. You can still do all the missions, but at a slower pace.