With the large-scale integration of distributed generation (DG) and power electronic devices, the power system exhibits significant low-inertia and time-varying characteristics. Traditional thermal-energy storage joint frequency regulation strategies, typically relying on fixed parameters, struggle to adapt to dynamic fluctuations in system inertia and lack refined consideration of the full life-cycle economics of energy storage. To address these issues, a cooperative frequency control strategy for thermal-storage systems is proposed, based on online inertia assessment and adaptive deadband optimization. A hierarchical oordination mechanism is established: under small disturbance conditions, the energy storage system (ESS) acts as a fastresponse istributed flexible resource with priority, utilizing a low deadband to prevent frequent wear on thermal units. Under large disturbance onditions, the strategy identifies the real-time equivalent inertia based on the inverse solution of the system frequency response and adaptively adjusts the ESS's virtual inertia and droop coefficients to provide dynamic damping compensation. Furthermore, an improved life-cycle cost model accounting for cycle life degradation is developed. Simulation results demonstrate that the proposed strategy effectively suppresses system oscillations and significantly reduces comprehensive regulation costs, offering a techno-economically optimal solution for distributed energy storage participating in ancillary services and frequency governance in low-inertia grids.