This all from my prior knowledge and my opinion of revising the past.
Shawn's injury was major in the history of the WWF in several ways.
1. Obviously the loss of HBK on the card
2. Austin was given the chance to be THE guy, and we saw how that went
3. Triple H had a chance to lead DX and himself to superstardom
4. The Rock came to be a major player in a very fast manner
Much of this was what helped the WWF beat WCW, but if Shawn would have been hurt, would any or all of the above have happened?
My vote is a resounding YES.
If he didn't get hurt, Shawn would probably have been just as big of an a**hole as he had been up to the injury. He couldn't have stopped Austin from getting to the top, and he didn't. However, he could have stopped, or at least stalled, Rocky. Yes, The Rock was groomed to be a star for a long time, but if HBK was around for all that time (see next paragraph), he would not have been pushed to the moon until around late 1999 or early 2000, assuming that Austin still left due to the neck injury. Before and during then, he would have found a way to put Triple H above him at every possible turn, which would have led him to stardom.
I do feel, though, that Vince at some point would have made Shawn take time off and sent him to rehab, taking him off TV. He would have b*tched and moaned about Austin being ahead of him like the prima donna he was and would have gotten Austin and Vince angry in the process. His drug problem would have spiraled out of control and his self-perceived image of where he should be on the card not being the same as Vince's vision would have exacerbated the problem. At that point, whenever Shawn being taken off of TV would have been, The Rock would have jumped to the top of the card and become a major player fast.
In the process of staying around, Shawn would have made two new enemies (Austin and Rock) after having ironically getting rid of one (Bret) and he would have turned his life into a horrifying tale along the lines of Scott Hall or Matt Hardy. However, IMO not much would have changed and he would still have had just as big of an impact on the business as he did in leaving, but in radically different ways.
And for those looking at the "Shawn would have gone to WCW" argument, if Shawn didn't get hurt, there is no way that he would go to WCW. Let's not forget that while Nash and Hall were there, so was Bret, who would have beat the sh*t out of him on his first day on the show IMO. They could have protected him all they wanted, but as long as Bret was under contract, he was part of that organization, and they weren't about to release a guy they had paid $9 million for. Add to the fact that HBK was a guy Vince supported and had a big contract with, and you see that the situation would have not happened, at least as any of us would see it happening.