SOCOM wanted an asset and decided on a solution crewed by aviators with the required experience, they turned to a fixed-wing aircraft with experienced fixed-wing aviators. That turned out to be the Super Tucano and Naval Aviators, I think with some USAF guys in the mix too. If rotary wing was the solution they could have easily turned to the 160th but they didn't, since that wasn't what was needed. End of story. Your comparison to the OV-10D's in Desert Storm shows how little you realize that newer sensors and weapons have changed the game for an asset like that, making them much more survivable and lethal. Nothing else in the US military can bring as much ordnance as quickly to the fight as fixed-wing attack/bomber aircraft and the A-29 was supposed to be a tool to help getting that ordnance get to the guys that need it. No one is saying we need to buy a bunch of them, just a few to fill a niche for a particular mission at a pretty low cost.
To get the thread back on track, the F-35 is the one fixed-wing combat aircraft that makes sense for the the USMC to buy. It fits most of their needs, it is even planned to fit in EA sooner or later, and since the Marines aren't about buying specialized aircraft for everything it is the one that can fulfill the combat role the best. I think the mix of F-35B/C will be a political one will ultimately be made at the Secretary and Congressional levels.