Curtailment is sort of free but it's also not. Particularly for a renewable source that needs it's uptime to be as much as possible in order to make financial sense. - Either way if supply can't match demand there is waste and this waste has to be included in the costing of the generator of the waste. All we are currently doing is moving the cost from the generator to the consumer. Also the RET is paid on output so if the output is not required or causing problems curtailment will reduce the RET cost, this then allows that money to build in places where it does make financial sense.
" Cheaper to build capacity that can be ramped up and down " ? Not so sure about that one ? This concept is a key principle in lean manufacturing and most businesses try to reduce storage and match production to demand as best as possible to reduce working capital.
I'm not suggesting that the storage has enough capacity for the whole period that the particular generator is offline. However, storage that takes the immediate intermittency away for frequency management and also helps with either end of the duck curve is crucial I think. If talking about solar and the generator wants to pay for the storage that is their decision. If the ends of the duck curve are a problem to the grid the costs should be borne by the generator then they can choose the best outcome. Same applies to any source of supply imo.
You're example of SA isn't quite right I reckon. Depending on the type of storage you have available to you, I think this could be managed in a modern grid. It might almost mean paying people to take energy and store it in their cars or houses but I think going forward that will happen. I think this shows my point from a great website https://opennem.org.au/#/regions/sa
There is already plenty of modelling going on to predict and manage wind intermittency. It might take a decade or two to get it perfectly right but that's ok. We have enough fossil backup to get us there. I think the modelling is flawed and over time and as more wind is added the reasons will be more evident. Imo we don't have 1 or 2 decades to sort this out and if it will take that long we should replace some of the older fossil now with the best available reliable supply and that excludes wind only or solar only that is not able to supply like for like.
