[franciscop]

We already have a real-world example of that[1], which is in medieval times in China, the land was a lot more fertile than in Europe, and with a crop also more nutrition dense (rice). So what happened there is that, instead of achieving something similar to the Shire, the plots of land became smaller per family, to achieve again an equilibrium where each family had enough to eat for themselves, but not a lot to spare. So more fertile/nutritional crops, while it would def help the Shire, would not be the whole answer without some sort of population management.

[1] https://acoup.blog/2020/09/04/collections-bread-how-did-they...

[Perceval]

Yes, that's one example, but it's not the only way that species adapt to resource availability.

Ecologists have hypothesized r-selected and K-selected species as differing responses to resources: https://en.wikipedia.org/wiki/R/K_selection_theory

Homo sapiens may be r-selected. Different cultures of homo sapiens may lean more toward r- or more towards K- (this is part of the thesis of Eric Jones's The European Miracle https://en.wikipedia.org/wiki/The_European_Miracle).

Insofar as Tolkien's work is a fantasy and Hobbits are not real, would it strain our credulity to imagine that they are K-selected?