Sure it could be a useful show of solidarity but I'm skeptical on the hypothetical conversion rate of these petition signers to actually quitting to follow Sam to Microsoft (or wherever else). Maybe 20% (140) of staff would do it?
If the board had just openly announced this was about battling Microsoft's control, there would probably be a lot more employees choosing to stay. But they didn't say this was about Microsoft's control. In fact they didn't even say anything to the employees. So in this context following Sam to Microsoft actually turns out to be the more attractive and sensible option.
Maybe. Microsoft is a particular sort of working environment, though, and not all developers will be happy in it. For them, the question would be how much are they willing to sacrifice in service to Altman?