In the US, this is FUD. There may be isolated instances of this happening which makes everyone spread doomsday news, but I really don't see this happening at every single place in the US within the next 20+ years, which falls out of your 'pretty soon' estimation.
You'd lose that bet in a big way. I'll take my winnings in cash, thank you very much.
I've yet to find one that doesn't accept cash, even in a major US city close to tech campuses. With the exception of a few small restaurants, the local self-operated car wash that kept getting robbed after hours, and a few farmer's markets (usually also at incredibly inconvenient times), I've yet to find a place that doesn't take cash. I've been to multiple restaurants in the past month that only accept cash, though.
How much?
No company refuses to acknowledge you before you swipe a card. Hell, you can't swipe a card until an amount is provided with only a few exceptions (gas pumps).
No food truck do you walk up to, swipe your card, and then give your order.
It is perfectly legal for a counterparty to decline to enter into a sales transaction with you because they don't like your proposed payment terms. They are not required to accept cash (or anything else that they don't like.) That is not satisfaction of a debt owed; no debt has been created, because they refuse to give you anything in the first place.
The "legal tender for all debts" part means that _if you already owe someone money_ for some reason, and you offer them cash, they cannot refuse the cash and then pursue legal action against you for not paying the debt. The court must rule that your offer of cash is sufficient to satisfy the debt.