Why Hand Wringing about Defensibility Is Probably Pointless:

• analyzing how product value grows as a company scales is really hard -- this generally involves reasoning about conversion properties of an adaptive feedback system
• analyzing how product value is impacted by competitors' scale is really hard -- also adaptive feedback system
• analyzing competitive equilibrium is really hard -- autonomous differential equation
• stochastic nature of serendipity and setbacks are really hard to bound
• all this shit is dynamic based on macro factors, team, cyber threat environment etc.
• market growth trumps all this stuff anyway, as fast growing markets create new white space which totally changes the value functions

I've now read a lot of the key papers on this stuff, and most of it is either post hoc, or makes mature market assumptions which I just don't think are all that relevant with early stage tech startups.

The tl;dr is invest in fast growing markets.