How is the parent comment's guess about 100% of people having email useful?

That it is true, and a communication system?

But if many don't use email much, how does it help? I barely check my personal email. And no one in my normal life knows my business email. Once in while a family member or friend will send something unimportant via email. Either I never see it. Or I see it a week or month afterward. My career relates strongly to the internet too.

If you don't respond to email and I don't communicate with you in person or on the phone I assume you are uninterested personally or professionally and take my friendship or business elsewhere.

Answering email is really a pretty normal expectation.

So is facebook for friendship lol

If you ask for their email and they offer to friend you, and you decline, should they assume you don't want to be in the loop for things?

Your arguments for email as a platform equally apply to facebook. Yeah email is decentralized and oldsr, but that stuff doesn't matter to regular people who want the easiest tool for the job.