So, does every business owner, regardless of the size of the business, now require a security guard to satisfy this duty?
Yes. If they want to operate in a dangerous environment.
Let me put it to you this way. You are out late at night in a bad neighborhood. You have every right to be there even though there is a strong chance you will be accosted. You also have a legal right to conceal carry for your protection and indeed your piece is oiled, clean and loaded.
Do you leave it at home knowing that if (when) something bad happens, you can always call the police?
If you would’t do that, if you would provide security for yourself, then why are you denying McDonalds the same opportunity to provide security under the same circumstances?