You explained your point very nicely and I agree with everything you said.
I think there is something to say about a company culture when it comes to using external services. You mentioned that this is how you do things and that it’s working for you. I appreciate that. We tend to not use external services (or many external libraries for that matter) for trivial stuff (trivial being something like leftpad). I believe this is mostly related to the personas in our case :)