The reason manufacturer interest in GT3 is so high is, is because it's a buisness case. They make money selling customer cars and get exposure by the odd faux works team or factory driver in one of their customers cars. It's the same as Rally2. Once you push that customer centered product to far up the food chain, manufacturers start to go crazy and the privateers start dying due to cost. Once the customers are gone the manufacturers loose their business case and leave. Just moving to a customer supported format is not valid solution to fix the current problems in rallying in my opinion.