There is just one Stone Cold Steve Austin. I mean I respect Michaels and all, but he could surely take as many years off and it wouldn't make much of a difference to the business. With such a thin WWE line up in 98, Austin become the biggest draw the business has ever seen. If in 99 when he got injured, he took the time off and decided to come back four years down the line, it would be a killer decision from a business stand point, which I think even Austin knows. And if that had happened not many from the new era would have cared for Austin. Austin's greatness was in the fact that the fans that loved him, loved him just too much. If he had gone, they would have surely not accepted him back that easily. And add to the fact, that he would be quite old too.
The WWF then (in 1999) needed Austin and therefore would do anything in their power to get him back on track asap.