There is one other thing apart from cost that I’d be worried about, and that’s aerodynamics. Building an airframe that can house a lift fan, a huge single engine, and weapons bays, and that can perform slow, low attack runs like an A-10 means compromises, so you get a plane that has problems with climbing, turning, can’t supercruise, doesn’t go very far and doesn’t get close to reaching Mach 2. Which mostly doesn’t matter much if you can avoid detection and have the edge in situational awareness, but what if the enemy can find your appropriate location (via long-wave radar, for example)? Then you have to deal with interceptors that know quite well where you are (in a bubble of maybe 2–3 km in diameter, good enough for IRST) and that you can neither outgun nor outrun. I think that could possibly get nasty.