You're talking about Lodash. But the remaining issue is of locality. If you use a package as a standard library, you still have to send that package to a client

You're talking about Lodash. But the remaining issue is of locality. If you use a package as a standard library, you still have to send that package to a client for them to use your app. Whereas a 'real' standard library would already exist as part of their interpreter/browser/operating system.