(Part1, Part2, Part 3 of this series of blog posts, for anyone who needs some context)
I got some time to think on a plane and distilled down the rationale for using java in a startup to this question.
"Is your startup going to need "average" programmers before your company makes significant amounts of money and/or gets funded?"
"Average" could be interpreted different ways - "dumb" (either absolutely or relative to alpha coders you do have), "low energy" (due to lack of time to focus on programming/ wanting to go home after 8 hours of work/ whatver), "low experience" (hire people with no programming experience), etc etc.
But all said and done, if you are going to need many average programmers on your project *before* you make (decent amounts of) money, then the decision to use Java (or C# or whatever the "mainstream" language is) is probably justified.
The question is a bit more subtle than it looks and has some circular reasoning baked in, but it serves to encapsulate all the reasons people select Java as a main programming language in a startup and can probably serve as a rule of thumb simplifying approximation.
The bigger question is what exactly does it mean for a startup to depend on "average" programmers, (I can't think of a convincing example of such a software startup, but I am sure they exist somewhere) but that is a question for another day!