The future power grid will integrate large shares of variable renewables, such as solar and wind, through inverter-based solutions. These developments change the power grid's characteristics owing to lower shares of classical synchronous generators and result in weaker, low-inertia power grids that challenge their secure and robust operation. To deal with this unsolved challenge, there is a potential for small modular reactors (SMRs) to provide system-bearing ancillary services, such as uncontrolled physical inertia and short circuit capacity, that can enable large-scale penetration of renewables. This article shows that SMRs can avoid the need for synchronous condensers to run renewable-rich power grids, thereby reducing ancillary service costs by approximately $20/MWh and simultaneously ensuring higher grid strength and inertia. Consequently, running renewable-rich power grids with SMRs as a backbone can be a competitive grid-forming solution in the journey toward a deeply decarbonized power system.