Good reality check.

I work on a product, a sort of web app, that was built several years ago, way before Figma (or even Sketch). I have found that it is a fool's errand to attempt to recreate the codebase components in Figma. The best I shoot for is when I redesign something, or design a new feature, that I use the heading styles, color, and font specs from the codebase, and work from there.

The existing pages are marked up in different ways by each successive FE dev that comes on board, so in reality, even the codebase doesn't have a "single source of truth." Add in that the html is now a mix of Bootstrap and Flexbox. So the "design system" is a pastiche of a style guide and components that roughly match the user facing designs.

If you are designing an app from scratch, a design system is useful. In many real world situations, not so much. You just have to do the best you can with what you have.