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Canada could write a law today and put it on the books that extends dual citizenship to every human being residing in North America. But that changes nothing within United States' borders. Not a single American citizen would lose their citizenship (natural or naturalized) as the result of a legislative act in Canada.

However, if an American citizen *chose* to take advantage of such newly offered dual citizenship in Canada by voting in a Canadian election, or skipping the Non-Resident Speculation Tax when buying Canadian property, etc, then the citizen has willfully enjoined an allegiance to Canada. Any children born to that citizen from that time forward -- even if born within the jurisdiction of the United States -- would be born of parents owing allegiance to a foreign sovereignty and would not be a natural born citizen by virtue of a dual citizen parent.

The Naturalization Oath of Allegiance to the United States of America (here), taken by your spouse when naturalized, is an oath to "absolutely and entirely renounce and abjure all allegiance and fidelity to any foreign prince, potentate, state, or sovereignty, of whom or which I have heretofore been a subject or citizen". A child later born to parents having enjoyed none of the benefits and exercised none of the responsibilities of citizenship in their origin sovereignty following that oath could not be declared by our courts as anything but a natural born citizen.

Clarification : A child later born within the jurisdiction of the United States to parents having enjoyed none of the benefits and exercised none of the responsibilities of citizenship in their origin sovereignty following that oath, the Naturalization Oath of Allegiance to the United States, could not be declared by our courts as anything but a natural born citizen.

If at first it seems so simple, then all of the sudden it is not:

“Under the right circumstances, you are allowed to become a citizen of both Canada and the United States, simultaneously. Many people enjoy the benefits of dual citizenship, allowing them to to travel back and forth freely, vote, and otherwise take advantage of the rights of citizens in both countries.

Before thinking about citizenship, however, you must find out whether you qualify to immigrate to either the United States or Canada in the first place. (We’re assuming that you’re already a citizen of one of these countries.) Citizenship is the highest immigration benefit you can obtain in either country, and getting to that point involves many steps.

There is no separate application procedure to apply for dual citizenship. If you’re already a citizen of one of these countries and become a citizen of the other without taking active steps to renounce your original citizenship, you are a dual citizen. It’s as simple as that.

(Note that taking the “oath of allegiance” to the United States, as described below, is not considered a renunciation of your Canadian or other citizenship.)”

https://www.alllaw.com/articles/nolo/us-immigration/dual-citizenship-united-states-canada.html

So dual citizenship is possible between the U.S. and Canada, and Canada grants citizenship to the children of Canadian citizens wherever they may be born. And U.S. grants citizenship to kids born in the U.S. with regard to the citizenship of their parents (even illegal aliens, as we unfortunately know).

Whether a person is a Canadian citizen by descent depends on the legislation at the time of birth. Generally speaking, any person who was born to a parent born or naturalized in Canada who has not actively renounced their Canadian citizenship is a Canadian citizen by descent (known as first generations born abroad), regardless of the time of birth.

https://en.wikipedia.org/wiki/Canadian_nationality_law#Canadian_citizenship_by_descent