Yup, this is the version that makes sense to me, with one addition: If a component is high cost, tightly integrated with the rest of the system you're building

Yup, this is the version that makes sense to me, with one addition: If a component is high cost, tightly integrated with the rest of the system you're building, and difficult to design, you will need to become an expert on it just in order to evaluate suppliers and make good purchasing decisions. Then (a) In order to become an expert on something, you need to do it yourself for a while; and (b) if you're an expert on it and have made it for a while, you might as well continue. Thus: for high cost, tightly integrated, difficult to design parts: build, don't buy. If a component is a major part of the value you're providing, it's sort of the same situation: you need to be an expert on it to safely buy it, and then you might as well build it indefinitely.