I'm not sure if there's a substantial dent in existing markets, but I would guess that as the Chinese market (the largest in the world) continues to expand, non-chinese makers will have a lower share of the overall global market. I don't think it has anything to do with open source though.

Not because open source but for not joining forces as a german industry and fighting each other to prevail by themselves meanwhile the chinese were using their technology (mostly sent to China by the german automakers) to produce cheaper and better cars (EVs)

Eh, kind of, but not really. They do have some open source initiatives here and there, but OSS is not the differentiator. They are just better executers, IMO.