Just because they already spent the money yesterday, it does not follow that the best decision today is to just carry on as if was still the correct decision. Yes they cannot get that 70B back, but if they have to choose between:

1. a long dragged out distraction over decades trying to make it work

2. a painful but quick 40B write down and the ability to refocus the company on better projects tomorrow

.. then they are, quite rightly imo, going to pick #2. In fact I would assume this going to be the next announcment.