Plenty of teams have a component library and call it a design system. The two look similar — a package of buttons, inputs, and modals — but they behave completely differently under pressure. A library answers "what can I render?"; a system answers "what should this product look and behave like, everywhere, forever?"
The shift is architectural
A library is a collection of endpoints. A system is a pipeline: foundations feed tokens, tokens feed components, components feed templates and platforms. When the layers are explicit, a change at any level propagates predictably instead of requiring a manual sweep across fifty screens.
This is also where headless architecture earns its keep. Separating behavior and accessibility from visual skin means one accessibility review serves every brand and theme built on top of it.
Systems have operations
The real tell is operational: versioning policy, deprecation paths, contribution flow, release notes, adoption metrics. A library ships code; a system ships guarantees. Teams build on guarantees.
If you're maintaining a library today, you don't need a rewrite to make the shift — you need to name the layers you already have, put contracts between them, and start treating consumers as customers.