Design systems really show their ROI when they help enable a large scale change. Think rebrands, mergers, brand updates, etc.
It's also the largest test of a design systems effectiveness. Most of the time a DS is used to improve the standard product release cycle. A major event like a rebrand shows how usable your design system really is.
A change that could take months or years now is a change that takes mere weeks. As these large changes are often unplanned, the time saved is an extra win for roadmaps.
In summary, design systems prove their value most in how well they prepare your organization for change.
https://superfriendly.com/design-systems/articles/design-systems-prepare-you-for-change/