If Redis had some big product people used, and their Redis server was just a middleware component, it could be a great side business where they sell support, but not the primary business.

All the other big players like Google/Facebook will open source those middle pieces to help people who will make apps that will most likely have to interact with their platforms. They'll never release the source to actual end products.

Are there any examples where the middle-tier stuff is successful on its own as a business model? I guess you could say Redhat or Canonical, but even then they're developing a lot of commercial stuff around the core open source operating systems/distros .. things like Landscape. Also they got in early, years ago, and might be able to build a better foundation than more recent companies.