That's exactly what my point is: This is way too complex to boil down to a simple rule of thumb to apply to all cases. There are arguments and counter arguments (and real-life examples and counter examples) ad-nauseum.

It's better to say: "Hey experienced people: How would you decide whether to introduce an external dependency or roll your own if the immediate requirement at hand seems small enough to write on your own in half a day?". Then we can get into a fun conversation that will boil down to "It depends."