Signing on to a Star Wars project is a massive commitment. Years out of one’s life. An intense period of preparation, then relocating to England for the duration of the shoot. One reason Abrams initially balked at taking on TFA was either be away from his family or uproot them and bring them along.
Getting sacked from a Star Wars film is probably as mentally devastating as losing a presidential election. Even more so if you’ve been a life long fan. Who in their right mind is going to want that level of stress knowing the axe is constantly hanging over your neck?
There’s no doubt it’d be a crushing disappointment. But for many directors, Star Wars is the reason they wanted to make movies in the first place. Even with the threat of the axe, I’m sure many would hate themselves if they didn’t at least give it a shot. I know I would, anyway, if I were in that situation.
Ultimately too, what’s worse, getting kicked off a Star Wars movie before you start production? Or making a shitty Star Wars movie and having everyone hate you forever? Because it’s quite possible Trevorrow was heading for the latter.