Had Bret done the right thing and laid down for Shawn, I don't think much would have changed.
Shawn was destined for the Taker match since SummerSlam. He still hurts his back.
Bret was still leaving for WCW, although his debut would have been different as he would have had zero reason for getting involved in the Sting/Hogan match. WCW would have still had no idea how to use him and he'd go to ground.
Vince was already building up to his Mr. McMahon character. Austin had already stunned him, he was mentioned more and more as an authority figure despite Sarge being the commissioner at the time. Not needing to do the Bret screwed Bret promo might have delayed it, but it wouldn't have stopped it.
HBK wouldn't have appreciated the gesture, (then) and it wouldn't have changed his attitude. Even if he had, he only had two more months until his injury and one more match after that, so any impact on him wouldn't have been a long term impact.
Now, there is one area where a change could have been felt. In late 98/ early 99, WCW was bleeding money. Had Bret been welcomed back with open arms, he might have been willing to get out of his WCW contract early and go back home. He was miserable in WCW, and I think he would have gladly given up the extra cash to go back to where he was appreciated. Had that happened, Owen doesn't die, Benoit doesn't have his two great matches with Hart that show he's capable of being a near the top guy. The title belt isn't vacant in January of 00, Benoit (because of the lack of Hart matches) isn't viewed as a worthy contender and doesn't get the belt. He has zero leverage, along with the other radicals, so they don't jump ship.
I think WCW still goes under, although at a slightly later date (1-2 years later). Being later, Hogan/Hall/Nash/Goldberg/Flair/Sting etc have all had their contracts expire and could appear during the "invasion" angle. With that kind of star power on the WCW side, there is no reason for any ECW involvement (it would have closed at the same time as t originally had) and no reason for Austin or Angle to jump to their side.
You would have had Hogan/Sting/Flair/Hall/Nash/Goldberg/Benoit/Eddie being the top part of a strong group going up against Austin/HHH/Rock/Bret/HBK/Angle/Jericho. The mid-card and jobbers would have been better, no Kronik vs BOD, could have had the Outsiders vs BOD. Benoit could have been what Storm was trying to be.
The biggest possible impact of Bret doing the right thing, was WCW could have lasted a bit longer had he left WCW early to return "home" and the Invasion would have been much bigger. Other than that, not that much would have changed over the long-haul. I think the much larger "butterfly effect" happened a month later. Had Sting beaten Hogan cleanly (as the storyline arc called for), Outsiders kick out Hogan, Hogan/Sting take on the nWo and vanquish it in 98. WCW's entire history changed at the point and in all honesty, wrestling history could have changed. Without the dragging nWo storyline and lack of any other big things in WCW during 98 (goldberg aside), they might have kept a little closer in the ratings and AOL/TW wouldn't have felt it necessary to kill it as soon as they did.
