That argument doesn't seem to be aging well.

I'd say that good [emphasis on "good"] coders can write very secure code. There's fundamental stuff, like encryption algos, that should be sourced from common (well-known and trusted) sources, but when we load in 100K of JS, so we can animate a disclosure triangle, I think it might not be a bad time to consider learning to do that, ourselves.

It's impossible to tell how that argument is aging since we don't have the counterfactual.

So if you can't trust your developers to manage dependencies, and you can't trust them to write dependencies... why have you hired them at all? Seriously, what do they actually do? Hire people who can program, it isn't hard.