Yes, but would you believe that a condition of some of our early investors is a promise not to pre-mine?
Yes, sure. As I outlined above pre-mine is so much hated in the community that it's a smart move for your early investor to demand absence of pre-mine.
If the project is open source, then someone could fork the chain immediately with 0 pre-mining and win everyone over.
It depends on what is the motivation to switch. For example, if pre-mine doesn't increase initial mining complexity, then there is a little incentive to switch in order to mine more in the beginning. Also will forks develop further? If I know that there are more features, infrastructure and improvements coming on the initial chain, I would think twice before switching to fork.