Maybe I'm in the minority, but I've never considered Crowder to be much of a leader. He tends to be the most vocal about his insecurity regarding his role on the team and potentially being replaced by a superior player (i.e., this exact scenario). Guys like Smart and Gerald Green, despite their flaws, bring a consistent energy that translates well in getting their teammates going and keeping things together on the court. Add in a guy like Horford who brings composure and patience (and I'd add Bradley to the list) and the team has plenty of glue in its current formation to survive a significant departure like Crowder. Even if Bradley were traded away, I think we have enough here to keep the general chemistry together to overcome any internal obstacles.
The other difference is that Perkins was a unique player for our team. He played a position that is hard to fill and he did so in a way that was impactful. We lost an enforcer in addition to our starting center when we traded Perkins. Crowder plays a position that is easy to replace and his production for that position isn't remarkable either. Considering we're in the running for two players that are significantly better than Crowder and play his respective position, I don't think it's a loss we should be all that concerned about.