99.9999% of people do not understand that point about inertia, you are one of them. and no it was not problem with spain, problem with spain was badly selected and configured inverters. if someone says otherwise he is liar.

" with old-school power plants, you don't need a lot of that stuff to stabilise the grid."

you just need proper sizing of renewables inverters + firmware update...... so no you do not need to have inertia of huge mass in turbines. also 99.999999999999999999999999999999999999999999999999999999% of all problems stem of peoples need to regulate grid to flat line for nonsensical reasons, IF you have slight artificially made "fluctuations in grid" which are generated by all inverter synchronized and planned ahead, there is no problem. grid has to have "pulse". THAT is decentralized / new grid. what you are describing is Stanley/Westinghouse grid. so mixing is resulting in nonsense.