I will always believe that if Bias had not died Bird would have won at least two more championships, and quite possibly three, which would have given him a total of six. Not only would Bias have been a major contributor to the team -- similar to Worthy for the Lakers -- he would have taken a lot of the stress off of Bird, which would have allowed him to stay healthier and play several more seasons.
Had those things occurred, which they very well could have, Bird would likely be regarded as the greatest player of all time. Even without those things occurring many consider peak Bird to be the highest level any basketball player has attained.