How many, over the decades, have fallen under "likely to succeed"?

None.

e.g. according to scientists/"experts", investors, project leaders, etc. Whose estimate gets used, anyway?

Whoever is making the decision. That's how decisions work. Said person would use whatever information is relevant to them. They will then decide whether they need to take action to prevent the destruction of all things good and light or whether they will take action to prevent someone who they believe to be intending to kill due to paranoia.