How are the OSS contributors, who initially provided IP under the premise of it being open source dealt with in these cases? If I were a major contributor to a package like Elastic or Redis and it was moving to a more commercial model, what stops me from suing Redis or Elastic and going after a share in the profits?

I guess the standard approach here is to release a new version with the new licensing model, and take all of the open source contributions and package them into a new commercial product. And anyone, perhaps even a contributor could do this, but the company is best positioned to do so and extract value from a bunch of free work that was contributed under an entirely different premise. Maybe there's room for a new OSS licensing model that prevents this from ever happening.