You can outsource everything, but outsourcing critical parts of the company may also put the existence of the company in the hand of a third-party. Is that an acceptable risk?

Control and risk management cost money, be that by self hosting or contracts. At some point it is cheaper to buy the competence and make it part of the company rather than outsource it.

I think you and I simply disagree about your database being a core/critical part of your stack. I believe RDS is good enough for most people, and the only advantage you would have in self hosting is shaving 33% off your instance bill. I'd probably go a step further and argue that Neon/CockroachDB Serverless is good enough for most people.