From experience i know that one bad bump can cause an injury, one bad botch can end a career. If you have a previous injury you are overly cautious with it, especially in the ring.
I have an old knee injury in my right leg, and when i wrestle, whoever i go against i make sure knows, so that they know never to 'work' that leg.
Even with the precautions, i have ended up hurt with that knee from a botched figure four leg lock.
If Austin even believes there is a remote chance that he could get injured, especially with his neck/spine. Even if he wants to, he is going to avoid it unless he can be assured perfect safety.
So basically, as much as the fans want Punk vs Austin, its highly unlikely to happen. I wish it was different though