I have been in too many startups that died because they lost or failed to recruit one particular person---or in the case of LMI survived (for a while) because I recruited one indispensable classmate who made their LAMBDA processor work---to believe this.
A reverse example would be the receiving clerk at Intel who single-handily almost killed them as mentioned in Chringley's book.
In my experience, just about every founder of a startup is "indispensable" after you shake things out a bit---make a single mistake with any of them, and you're not likely to survive.
I suppose that means this is an "appeal to authority", but, still, if a person critical to making software work is gone, and the company fails specifically because they can't make their software work, what else am I to assume?
(Needless to say, sometimes I've been that person.)
FWIW, no one is indespensible in a startup; some are merely harder to replace than others. I'll bet that "original idea" guys are getting equity even though they cease to be indespensible.