Of course we can never know for sure, but we can certainly draw very educated guesses and conclusions.
For starters, the argument that "perhaps some" of them would have behaved the same is no where near as nice (in open source terms) as "absolutely all" behaved this way and will continue having to behave this way.
We can also analyze the behaviors of some of these companies. Just look at Safari for example: the open sourcing stops precisely where the licensing requirements stop: the engine is open source but the browser itself is closed source. Apple has unquestionably contributed the most to this web engine, and they are unwilling to contribute any more beyond their legal requirements.
Additionally the effect this has created in the web world is pretty powerful. Companies are now in a position where their choices are investing a tremendous amount of work in creating a new web engine or using the best existing web engine to be competitive but having to contribute all their changes back. This is particularly true in the mobile web space where WebKit is essentially the only engine that has proven itself capable of providing a desktop caliber experience. In terms of open source goals, there could be no better situation. You basically have to use an open source engine to be competitive, and the most competitive of these engines requires you to contribute back to the community in a space that is dominated by commercial investment.
For apple I think the theme is engine / skin not what the license requires.
osx : darwin :: safari : webkit
Notice how the relationships revolve around the engine vs skin, not what the GPL requires. In other words, apple protects what makes them apple, not the underpinnings, at least in these examples.
Perhaps they wouldn't all have contributed code back.
On the other hand, maybe yet more would have used the code, and some of them would have contributed code back.
Copyleft vs. copyfree is not a zero-sum game. Choice of license doesn't only affect the restrictions placed on those who use the code; it also affects the likelihood that someone will use it at all, and we really don't know how many people avoid using some code for a given license and might have contributed their improvements to the community.
For starters, the argument that "perhaps some" of them would have behaved the same is no where near as nice (in open source terms) as "absolutely all" behaved this way and will continue having to behave this way.
We can also analyze the behaviors of some of these companies. Just look at Safari for example: the open sourcing stops precisely where the licensing requirements stop: the engine is open source but the browser itself is closed source. Apple has unquestionably contributed the most to this web engine, and they are unwilling to contribute any more beyond their legal requirements.
Additionally the effect this has created in the web world is pretty powerful. Companies are now in a position where their choices are investing a tremendous amount of work in creating a new web engine or using the best existing web engine to be competitive but having to contribute all their changes back. This is particularly true in the mobile web space where WebKit is essentially the only engine that has proven itself capable of providing a desktop caliber experience. In terms of open source goals, there could be no better situation. You basically have to use an open source engine to be competitive, and the most competitive of these engines requires you to contribute back to the community in a space that is dominated by commercial investment.