You are getting to the heart of why this word was replaced in the Scrum Guide. You can find some context to the change here (http://www.scrum.org/About/All-Arti...to-Scrum).
What do you do if not everything gets done in the sprint?
As soon as this situation becomes a possibility we make sure to inform those who care. Particularly the Product Owner so we can discuss our options.
We might also explore the why this occurred during our Retrospective (or any time really). This is particularly important if the current process and environment doesn't surface this information with an opportunity to make adjustments. For instance, if it only became clear in the last few days of the sprint that something might not be fully Done, what experiment can we come up with to surface this information more readily. It may be a simple gut check during the Daily Scrum or it may be a change to reduce serial task execution.
Either way, you are right on: we cannot avoid the Cone of Uncertainty even within the Sprint. For that reason, we do not use the word commitment with all of it's baggage.