Some people rely on their relationships with other people. They build support systems, form groups, and take comfort in the company of others. Being cut off suddenly and with no means to reconnect would isolate many people. This would be extremely damaging from a mental health perspective alone.
In addition, so many people have reconnected with old friends, family, teachers. One friend of mine used Facebook to find her birth parents. That avenue disappears completely.
I don't know why you would say it's "good" for these companies to disappear. These aren't megacorps. It's small, local companies that rely on FB advertising. Bed and breakfasts, wineries, anything that depends on tourism. They're always hit the hardest. Walmart isn't going to care if they can't advertise on FB anymore.
Your comment shows a lack-of-understanding and compassion for your fellow human.