What inertia do you mean? Demand or supply? Sure, decomissioning anuclear plant takes decades but a coal plant can be shut down in about 48 hours I think and at that point produces no new emissions...
"Inertia" in the context of power systems refers to the kinetic energy stored by rotating generators (coal, gas, nuclear, hydropower).
If the system frequency (a real time measure of supply and demand) were to suddenly change (e.g. a power plant were to trip), these generators are still spinning, and dampen the rate of change of frequency. This allows time for the grid operator to take corrective actions.
Inverter connected plant such as wind and solar does not inherently provide inertia.
