Thing is, the decision whether to open source your code comes first, not last. GPL doesn't force it in any way, GPL prevents one from using GPL code if they don't intend to immediately release everything.
In other words, what would happen with GPL is that companies which don't intend to give the code back would simply go somewhere else instead.
In other words, what would happen with GPL is that companies which don't intend to give the code back would simply go somewhere else instead.