The question to the title of this post is absolutely. And there is precedent for this as well. And should Apple go through with giving away Mountain Lion and maybe charge on a biannual basis or whenever they figure the upgrade is big enough to warrant a charge.
And Mountain Lion is a bit upgrade from Lion. However, this is a sense of urgency in this. Apple is on a roll in both the mobile and PC market - growth beyond anything I expected. I'm not alone in this. Giving away Mountain Lion will add fuel to Apple's success in 2012.
More at On Apple.