Pau Gasol, Chris Bosh, Dirk Nowitzki, and Lamarcus Aldridge are among some notable bigs in the NBA who at points earlier in their careers have dealt with the "soft" label. I even remember Tim Duncan being accused of being soft earlier in his career.
I don't know if this label was ever fair to any of these top notch NBA big men. I do know that they share games that are more based on skill and finesse than on physically overpowering the opposition.
I also suggest that "toughness" is a trait that can be developed. I don't believe it's as finite a quality as one's pure athletic ability. While perhaps the softness label was always a bit unfair to the above players, it seems likely that they have in fact all been toughened up some by years of battling night in and night out against big, strong, athletic NBA bigs.
Based on the belief that NBA big men can and do get tougher after years of doing battle on the interior in the NBA, I have developed a theory that one of the main reasons that Danny has yet to get a rim protector or an interior tough guy to help protect the young front line of Olynyk and Sully is that he doesn't want them to be protected. He wants to play them minutes at the center position to give them a crash course in toughness.
Hopefully the strategy pays off. When Kelly Olynyk eventually is the starting power forward for the Boston Celtics' next contender, it will be as a finesse, skilled, offensively inclined, stretch four. I am hoping that his early years being forced to play the bulk of his minutes at the center position against big, strong grown men will help make him a tougher player when he reaches his full potential.