It's not true that a rotating windmill blade has zero rotational inertia.  Sure, one 5MW wind turbine has less rotational inertia than one 500MW steam turbine, but does it have one hundredth as much?  In addition, if your reliance on large centralized generators goes down then so does the likelihood of the problem you describe.  Wind turbines may fail as frequently, per MWh, as standard steam turbines, but that implies more frequent but less severe outages.