It's quite easy to spot a stage 3 Phone-zombie. They are easily distracted even in conversation, shifty eyes, can't focus on tasks, bad sleep habits. This is a huge limitation in life and a pretty crappy way to exist, in my opinion.
People who are rude in public are just unhappy with their lives because they haven't found deeper meaning, so they try finding comfort in becoming a phone-zombie.
It's a vicious cycle.
It's really amazing to me how all-in a lot of people are with their phones. I think that the global population would collectively lose their minds if all of a sudden the internet just wasn't there one day. I would be affected by it as well of course, but I am still aware of my surroundings and try to be polite in public.