It would, indeed, make sense to have standardised behaviour for the inverters (they can simulate inertia perfectly well, but a lot do not), and incentives to contribute that value to the grid. It's not a hard to define target if you just want something that looks like inertia.

And the root cause of the blackout in Spain is still mostly the subject of speculation. AFAIK there has is no official report yet into the details of how things went wrong.