The four freedoms go far beyond just the ability to fix stuff. It also includes the ability to redistribute copies and modified versions.
I think there are more options than just "open source it all" and "keep it all proprietary". You could, for example, only provide the source code (or parts of the source) under a NDA contract when requested and merge "community fixes" upstream, or something. I don't know what would work well in practice (not many businesses have experimented with it) but I'm fairly confident a model can be though of that works well yet isn't "open source" in the sense that we understand it today.
I think there are more options than just "open source it all" and "keep it all proprietary". You could, for example, only provide the source code (or parts of the source) under a NDA contract when requested and merge "community fixes" upstream, or something. I don't know what would work well in practice (not many businesses have experimented with it) but I'm fairly confident a model can be though of that works well yet isn't "open source" in the sense that we understand it today.