When shared components become worth maintaining

A small site may need little more than a few shared styles and templates. Maintaining a separate component library would be extra work, and there may not be enough repetition to justify it.

That changes when a button fix needs copying into several places, or two forms handle the same error differently. Those are specific problems a shared component could solve. I'd start with them and include examples from the product. Someone also needs time to maintain the components after the initial work is done.