Buddy...
They weren't idiots, it's not hard to figure out if you grew the same crop in the same place it worked out worse than before.
Not to mention this isn't a hypothetical, archeology exists bro
Don't think people thousands of years ago were dumber than us, humans have been unchanged for over 300k years.
300,000 years ago ancient humans were born just as intelligent as any of us born today. And they dealt a lot more with plants, anyone that grew the same crop on the same land for multiple seasons would (and did) figure this out.