The question is how CAS is performed. Yes, there always were slow low good ground support aircraft, starting from Stuka and armored IL-2 up to modern A-10 and Su-25 that can have great deal of punishment and get close to targets.
However what is common to Ju-87, IL-2, A-10 and Su-25? They can operate when you have local air superiority, otherwise they are canon fodder - this is right for Aces High and for modern combat.
Now the question is if you need to get close to the target low and slow? It is good question - if you think that you don't need to get low and slow than you probably don't need choppers like AH-64 & AH-1 - that are even more vulnerable than A-10 or Su-25 at modern battle field.
If you use this argument that A-10 and Su-25 are dangerous in modern combat environment than retire all combat choppers fleet - so I don't buy this argument.
On the other hand if you operate F-16 like aircraft in that role - you can always drop your bombs and go on equal terms against opponents, A-10 and other specialized type of aircraft would need air cover.
A-10 and other similar aircraft are very efficient in CAS and probably do stuff better than F-16 in their role. F-16s + drones + choppers can substitute them by different means - as everybody already do it.
Now it is question of budget and goals. Can you afford to keeps specialized aircraft or can't? For example USAF is only operator of A-10. Su-25 is operated by much wider range of countries (outside CIS as well) but I assume Su-25 is cheaper to build and operate.
So bottom line:
- So would retiring A-10 jeopardize CAS abilities - no - almost every other force does CAS without it, also it may look different and may cost more at war time in comparison to dedicated aircraft.
- Would be it more cost effective to operate F-16 and other aircraft types in CAS role in long term (not, I'm not talking about F-35) - it is a question to to USAF.