There was an interesting debate at one of the think-tanks in the US that I attended, about the A-10, and a fighter pilot started out by saying that the A-10 is an exceptional aircraft, and then went on about some of the missions it has done remarkably well in Afghanistan and Iraq. He then went on to share a hypothetical scenario where the enemy could have actually fought back and contested the air, the ground, space and everything in between. He said something like, the post-war debate and media narrative may well have been how the US Air Force suffered from last-war'itis (equally dangerous to next-war-itis) and completely ignored the operational realities the A-10 found itself in, and spent hundreds of millions keeping a capability operational that was totally unnecessary.
The A-10 debate in the US was settled in the late 90's, when the USAF quite publicly and formally acknowledged that it would be trading away, quite consciously and through an informed and disciplined process, speciality aircraft and the capability it brought in favor of a smaller force structure (forced onto it by the political realities of the post-cold-war US) and that going forward they would be placing their bets on Remotely piloted aircraft, protecting space based assets, and reducing the cost to acquire of the asymmetrical capability it had developed as a result of the second offset strategy of the 70's & 80's. What happened was that the US found itself in a counter insurgency, and a lot many A-10 pilots who had known no other type of war began to come up through the ranks feeling that this was the only war the US needed to prepare for. Couple that with the Congress shaving a trillion dollars of the ten year budget and we had ourselves a showdown between the USAF that wanted to preserve its future, and the A-10 community that wanted to preserve a capability that had served the US well over the recent past. It ultimately got resolved thanks largely to more money through a budget agreement, but the narrative the media drove home was that the A-10 community somehow won a symbolic battle.
I often quote folks that have done CAS and/or written about it in that "its a MISSION not an aircraft". With CAS you have different sub-sets, different requirements and different scenarios you find yourself in.
Some of them are -
* You need to get to the troops on the ground fast - so moderate-long distances but a lack of time : Here the A-10 is indeed inferior to any aircraft that can get to the area using a supersonic dash.
* CAS when the enemy's air-defenses have not been severely degraded: Threats can come from the air, from the ground in the form of fighters, SAM's, EA/EW disruption, and MANPADS and AAA. Here you need an aircraft that can either simultaneously deal with these threats and still provide support, or you need a very large package that opens the air-space up for a dedicated CAS aircraft to do what it has to do. Either way, you'll end up with situations where you don't have enough EA/EW assets to help escort your slow, low flying CAS aircraft, or you won't be able to position enough ARM's over kill boxes to take out pop out emitters. In this case, a self escorting, survivable aircraft definitly scores much more than the A-10 or a similar aircraft.
The A-10 will still do the mission if called to, but you build your force structure, doctrine and training around what is required to accomplish objectives with high degree of probability and frankly there is little guarantee that an A-10 irrespective of what the attrition rate is, can provide adequate close air support when the threat to it from the air, from the ground and from non kinetic sources is still high. If your probability of mission success is low, it really hurts the overall close air support mission and then forces your grunts to make other arrangements. Simply put, if air-support is unlikely to show up in the form of an A-10 for example launching a bomb or a missile, the infantry for example has to find other ways to obtaining that kinetic effect. There are options for example for lobbing an SDB from the ground if one can't be put up from the air, but you have to invest in that and hope that it is available as required:https://www.youtube.com/watch?v=kkkfE5yHejc
* Another kind is where you need persistence for both ISR, and in the support role: Here compared to an F-16 and F-35 the A-10 is generally superior given that the others have their loiter times greatly influenced by other designs trades that make them multi-role fighters. IN such a role a hellfire or an SDB armed Reaper, avenger or even a lower cost drone is even better because the ones available NOW can give you more than a dozen hours and soon they'll be able to refuel. Even conventionally armed bombers like a B-1 or B-52 are much better than the teens or the A-10 in such a scenario since they have a huge payload, and can hang around for a long time once they do arrive in the theater.
* COIN, or CAS in a low-threat, or no threat environment: Here is where your A-10 really does well. But a Scoprion can do perhaps 80% of the mission of the A-10 (all but going low and using the cannon) at a fifth or so of the operating cost.
There will be lot many more scenarios but what this highlights is that CAS is not an aircraft that does it all...it can be broken down tactically where a lot many characteristics come into play, characteristics that are impossible to put onto one aircraft at an acceptable cost, or frankly at any cost.
I think in a nut shell, the A-10 and similar aircraft largely now serve a niche role compared to the era in which they were designed. The Scorpion is also a niche aircraft. However, if the niche mission ends up being an incredibly important mission in terms of hours you spend doing it, at least over an decent period of time, it more than justifies the retention or acquisition of the capability. If the US pulls out of the ME, there would be no case for the Scorpion, or to retain the A-10 beyond 2020 for that matter. But as long as the US stays, there is both a common sense case, and a business case to be made that perhaps a lot of the manned missions of the F-16, A-10, and of the F-35 in the future can be offloaded to the Scorpion or similar aircraft to save cost and preserve the air-frame lives of the more expensive aircraft - With minimal if any impact on the outcome of the air-operation.