Interesting question that I think a lot of people think about, but maybe for only a few seconds once in a while. I'm not sure how deep this hypothetical goes, as the Internet is less of a thing and more broad idea comprised of many things. Like, if no internet at all for real-real, it would probably bring about extreme economic collapse in many areas, catastrophic logistics failures, healthcare systems crises, etc. But if the hypothetical has more to do with the social and entertainment content of the Internet, it would affect different people differently. Many people are addicted as you described, and would suffer a lot of withdrawal effects. Very painful in the short term, but most gaining something in the long term. But many others who are balanced with theirs of it actually get a productivity boost from those resources without overindulging in self-destructive ways. There are many others who have developed their core career competencies around the platforms and would suffer great loss to business or career, while others would gain new business and career opportunities from it.
I think because the social platforms grew so quickly and were so novel to how we've evolved to communicate over human history, Many have been ill-equipped to deal with its seemingly sudden injection into our lives. I have seen unexpected trends happening, albeit slowly at first, in the youngest generations. Many are developing self-limiting skills to deal with the burdens these platforms and algorithms put on them. I expect that to continue–I certainly hope it does. I believe in the resiliency of life to adapt to its environment, no matter how unfortunately painful a process it may be in the short-term.
A few years ago, if I had a magic "shut off social media for 1 year" button, I think I would have pushed it–not quickly or without thought, but I think I would have. Now, I'm not so sure.