Right. If you ask me every day for my chainsaw and I give it to you, and one day I decide that I need it and you cannot use it, it's your issue that you can't get your project done. Your projection was wrong.
Now, if you had reason to believe (say, a contract) that my chainsaw will be available to you every day for three years, yes you can count on it. Otherwise you cannot.
Perhaps we shouldn't have "temporary" tax cuts at all. Why is it that something that was good 10 years ago is all of a sudden not good now? It's not like the "permanent" tax cuts or hikes are really permanent: the government can revise the tax code fairly frequently.