(most?) common cause of failure for founders with ~10 customers?

Hmm probably cofounder fit? Relationships aren't always easy. Especially when there's only ~10 customers which means you probably don't have enough to be ramen profitable and there could be some disagreements about how much time each cofounder is spending on the business.

And yeah... this is also anecdotal from my own experience! Although I've heard this be a common cause for failure from many others.

I agree with this if the ~10 customers have no referral value.

For example, if I spend X amount of time/money on a customer who only delivers half that lifetime value, I'm doomed if they have no referral value. But if for each customer that delivers 1/2X in LTV, if they have an audience to share their success story with, the branching referral outcome could leads to 10X the initial time/money investment. So at that point the key is low touch scalability as the referrals come in.

I also think that when you have ~10 customers, it's good to spend a surplus of time talking with them, to understand them deeply and further refine your PMF internally. If during that time, certain customers become less relevant and are a big time suck, with no referral value, then it's probably a good time to let them go.