1. Knocking out Syria's air forces is actually a huge blow
They have a finite number of helicopters and planes, and Russia isn't going to resupply them now. In addition the rebels' hit and run tactics against air bases might not seem effective, but it also means destroying maintenance supplies, which Russia probably isn't going to give them any more of now either. Meaning that eventually they won't be able to get the helicopters and planes in the air.