First, it sends a message of inexperience in business, negotiation, and conflict resolution: 'I'm going to take my ball and leave' - it looks like an emotional overreaction without strategic thinking. These days you sometimes see powerful parties making similar threats - e.g., Uber threatening to leave certain markets. But those people have significant power and their tactic is really to demonstrate that in order to shift their negotiating position; usually they don't actually decamp, and GrapheneOS has relatively little power so that tactic doesn't apply.
As importantly, it sends the message that GrapheneOS can be pushed around and manipulated: A slight hint of a threat and they flee. Others will take note, and many will think the same of other FOSS projects, large and small - they are easily intimidated and dismissed.
Another reason people don't use these tactics is that they have other important interests besides the one under immediate threat. A requirement of anyone with significant investments that can't be easily abandoned - which is everyone doing anything of value - is to navigate in a way that upholds all those interests. You don't burn down the house to kill a rat. It can be hard and requires careful, deliberate thought and strategy.
One unmentioned interest that might appeal to GrapheneOS's leadership is the freedoms of people in France to create FOSS, and to individual privacy and security.
update: I told Gemini we made it to the front page. Here is it's response: