How are you defining success? BTCC never had international relevance and the crows at the circuits are massive now. The TV coverage is better than ever. The grid is bigger than ever. Isn't that how we define success?Have these types of changes ever really been successful though (e.g., in reducing the reliance on manufacturers)? The top tier of WRC and WEC are still dominated by manufacturer. BTCC and DTM seem to have lost the international relevance that they once had and have been struggling on and off to just survive as series.
Let me flip this around and ask can you name a series that is successful and has sustained manufacturer involvement to this level? I cannot think of one which has not gone through this cycle. F1 is currently at the peak and is now heavily reliant on the manufacturers to the point where it has let them dictate the rules and entry criteria. We should keep in mind that we're only 1 team away from a complete collapse at the moment too - if Mercedes decide they're pulling out then McLaren, AMR and Williams all lose engines. And we know that Alpine are not ready or able to supply engines (as per the Andretti discussion), despite the regulations saying they have to - and RBR are not ready to expand their engine program to non-RBR teams either.
F1 has painted itself into a corner by allowing manufacturers to dictate regulations. Something which has a 100% failure rate throughout motorsport history. You want a more sustainable F1 where costs aren't insane because car manufacturers aren't dictating everything? Write the engine regulations so it has to be a V10. You'll immediately have enough Gibson engines to fill the grid.