Nice write up! I would add another dimension to when you build on your own. If your business requires customization.Eg. build your own CDN with custom transport protocol(just came up with something)

A prime example of this that I've personally run into is operations tooling. How many times has logging infrastructure been built from the ground up? I can think of a company (which are pretty great, to be fair), whose core business failed but ended up spinning out their logging infrastructure as a standalone product. The logging product is actually really good, but it begs the question if their focus on building logging infra distracted from their core business.