You’re absolutely right. Now that the Watch 3 has data, who needs the iPhone ;)
In all seriousness, the addictiveness of products is a universal fact: cigarettes, TVs, sugar, cars, etc. Unlike these other products, you can turn off notifications on an iPhone for all but essential messages. I suspect that was the intended use of the notification system: all apps provide the feature of notifications, it’s up to the user to silence all those which are unnecessary (e.g. Instagram, Facebook, Messenger, LinkedIn, Shazaam, Podcasts, etc.)
Life is more than the confines of a phone. It’s a tool like any other.
You raise an important issue but the above had to be said.