"If it’s a core business function — do it yourself, no matter what". The question is, what is "Core Business Function"?
If you were an internal tech company in 2000s, hosting your own servers was a "core business function", as being a tech company meant that tech was core business function. However, as tech got more sophisticated, the domain of "core function" became more focused per company
Another example is the rise of fabless semiconductor companies. Even until recently, Intel had superior chip design and fabrication capabilities (thus intel's tick-tock strategy). AMD founder Jerry Sanders claimed "Real men have fabs" but AMD's spinoff of GlobalFoundaries look like a genius move now.
What you think of "core business function" is going to change over time.