How many of your assumptions about food logistics do you expect to survive the collapse of the gulf stream?

You honestly believe that we will return to a medieval era if the gulf stream is disrupted?

The gulf stream is predicted to collapse in about 40 years. You won't catch me making strong predictions about what happens that far in the future, but the gulf stream is responsible for keeping North America's farm belt temperate, aka, farmable. The North is expected to cool dramatically, and closer to the equator is expected to warm dramatically -- perhaps inhospitably so. Additionally, the oxygen levels in the ocean may deplete to produce mass extinction in another major food source. I'm not sure what all is implied by "medieval era" to you, but all together, it looks pretty damned bleak. Evolution doesn't turn on a dime, but the weather might.

"Medieval era" implies a regression to subsistence farming and the end of today's technological civilization. Even a severe disruption to food production would result in substitution of foods, not a destruction of civilization.

I’ve seen that movie.