Focusing on CAC early certainly has its merits. Of course, a MAC doesn't free you from iterating the product decisions. And of course, you'll loose a lot of early users because your Product Market Fit is not 100%, not even 70% from the start. The real entrepreneurial value in the idea of a MAC is that you throw money at customer acquisition from "day one". This certainly works only for products where you can afford to loose early users, like in B2C markets. I'm not sure whether it works for B2B markets, at least the initial value of the product needs to be higher to reduce the churn very early on.