You're right on all your points from a pure security point of view. We should be doing security as soon as possible. Unfortunately, the reality of building a startup is about finding product-market-fit. Entrepreneurs are not incentivized to do security early on. The fear strategy our industry is using for the last XX years has failed.
As security professionals, we need to help entrepreneurs and educate developers find a good balance between building a business and building good security practices. This is the goal of this checklist.
We can't expect developers to spend days implementing security best practices before even having a business.