If they already have the feature then it makes even less sense to not give it to everyone. Like I would switch my phone OS just if there was a simple way to disable internet access totally for apps.

I wouldn’t be surprised if the only reason this exists in China is because of some legal requirement or a requirement from local carriers. I’m sure Apple and the rest of the industry would rather not offer this as it would break many data collection and “engagement” strategies (you can disable network access for single-player games and not have any ads for example).