This is half wrong.

The fallacy is that a course of action that is still objectively worse is viewed as better because of past investments towards that goal.

Doing things you've started even though you're unhappy right now is called growing up. It's not a fallacy. In fact, the opposite is a fallacy: what feels good right now is always best.

So, if Arriane is halfway through her PhD and she realizes it's not as good as she wanted, it very well still might be a good idea to finish it. Or it might not. She needs to re-evaluate now, given that she can get a whole PhD for a half-PhD's work.