Having worked at two startups that died either because they could never attract skilled senior engineers or because they lost them, I think the number is quite large.
There're lots of ways for a startup to die and relatively few for it to succeed. The value of having a skilled senior engineer (or "rockstar ninja" as you put it, though that term has connotations that don't really describe the type of early engineers you want) is that they can see many of those failure modes ahead of time and avoid them. And when they do end up in a dead-end, they can quickly backtrack and fix the problem instead of wondering "what do we do now?"
There're lots of ways for a startup to die and relatively few for it to succeed. The value of having a skilled senior engineer (or "rockstar ninja" as you put it, though that term has connotations that don't really describe the type of early engineers you want) is that they can see many of those failure modes ahead of time and avoid them. And when they do end up in a dead-end, they can quickly backtrack and fix the problem instead of wondering "what do we do now?"