just my opinion, it is something new, not low threat but not expressly medium or above, it's an unpredictable threat level environment, most of the time the SAFIREs are small arms but there are MANPADS, medium level AAA pieces (57mm and up) that show up occassionally and the big wild cards are the remaining air defense capabilities of the failed or failing state and in the case of Syria, the very serious capes their allies (Russians especially) bring to the AOR
the air environment for what's being called the "Grey Zone" conflict, a mixture of not equal parts of war, tension, stabilization, chaos, crime, etc...
Is the A-10 the best weapon for CAS?
Right now absolutely, but the operational environment can and always changes. In the future maybe not and if we keep intervening in these Grey Zone fights we will likely need an A-X that is built for the unpredictable threat environment, unpredictable required effects delivery all while not punching a hole in the budget stretched thin from the weight of 5th gen acquisitions
What I would design for in a X-gen attack aircraft:
Reduced signature and self escort / air to air (BVR) defensive capability
Networked with integrated sensors for self-cueing, PID
Designed around delivering PGMs vice direct fire weapons
Long combat radius or endurance with AR flexibility, boom or drogue
UAV wingman or drone control from a backseater, a UAV tanker and/or ISR platform like the X-47 the Navy is developing
All those capes I just listed would likely make this hypothetical A-X unaffordable in this climate and may be more than is needed right now but considering how this first Grey Zone conflict in Syria is going, I think the argument can be legitimately be made for this type of attack aircraft.
Not a pointless effort to do CAS in high threat environment like a mission over Moscow but one where conventional military capabilities and threats are being used by competitors to shape the operational environment, either by denial, intimidation or actual use in limited ways.
Even though it was an abortion of an acquisition attempt, the A-12 Avenger II (or an updated version of it) is what I am thinking of for an A-X