Your claims are fallacious. Most people, i.e. 51%+ don't use USD or EUR. Many people also make international transfers. There is a use-case and your observations that zero people use btc are anecdotal at best.

Pray tell what kind of legal issues would suddenly arise if it became mainstream.

Sure, I can buy some btc and go buy something with it in some places. But where is my incentive to do that, rather than just go use my credit card or an ACH transfer? Btc is not more convenient, it's not cheaper, and whatever gain in privacy there currently is will not survive a transition to mainstream usage...although the first two problems ensure that there never will be mainstream usage.