Right, infact of the three true CAS plane actually in service, our own one (the AMX Ghibli) use just a standard M61 peashooter and has not any armor,
Fundamental requisite of a CAS manned plane is not the BRAAAP level (Su-25 was as an example just the third one in the arsenal of the SU in that regard) but the ability of fly at a low level and low velocity for a consistent period of time over the battlefield and being able to directly interact with the units on ground in a two way mode giving both information/reconnaissance than fire support for all the time of engagement.
AND finding a fitting substitute for all this requirements it is not so easy task:
Mig-27 had a gun even superior of that of an A-10 but cannot stay long and fly in any case too fast to interact with troop on grounds.
A-16 will have had the same problem, just worse as it had the wings and consequently the flying characteristics of a fighter, not even of a conventional attack plane.
An armed UAV can stay even longer that a CASplane but is too lightly armed and also the two way communication part is problematic : eyeball Mk-1 and an human mind in the cockpit itself and not in another continent are a must in this case.
Also because having a gun, even a small one on board is still very useful anyway: it was ascertained that performing (even a mock) strafing run has the advantage of pinning down attacking infantry over the whole assault line .
Still F-35 and Su-34 are a good compromise on that regard, from 4+ gen planes onward, planes sport a good Aoa , so they can keep a way lower minimum speed and so they can fit (partially) in the desired speed and quote needed for the role.
Yes, they are an overkill but a B-1B and Reaper team up , as it was done in the battle of Kobane will be even more so.
Modern optronics and targeting systems allow planes to perform close reconnaissance tasks and even use guns and rocket pods at a way higher quote than before, so its not even an urgent task to find a substitute (almost for russian Su-25, that have still a lot of flying hours in the belly, AMX and A-10 not so much).
I cannot envisage anyone sending either an F35 or Su34 out to do true CAS, in a peer to peer war, unless they are the last aircraft you have, and you need 24 hours to flee.
Likewise drones, have had some successes, but a drone is a fairly single mission piece of kit, it has one speed(normally pretty slow) relatively fixed altitude etc. Also of course you must have comms to the drone, unless you release it to hunt-kill, but I dont think you can then get it back if it doesnt find a target?
Most Nato forces will have drone countermeasures, and even for non-G20 its not too difficult to think of countermeasures you could immplement, lets not forget much of WW2 was done with manual tracking of enemy aircraft, radar was not commonplace, outside of the UK, at least in the first half.
With a manned aircraft, you can change tactics very quickly, from low height, to med or higher, Gulf war, started super low, with runway denial, and toss bombing, and quickly moved to medium altitiude, with LGB etc. In a conflict presumably with a lot of EW etc, being able to send a manned aircraft with alternate plans or targets, versus a drone you launch, and then lose touch with, is going to be interesting.
Certainly manned aircraft are good targets, but as always this is not chess, and there is not an AD unit every 10km of the grid. Good intelligence, stealth, drones, training, etc all come into it. Likewise the will to win, and the reason/location of the conflict.
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