There's yield advantage, but costs, too, in narrow rows as compared to 40-inch rows for corn and soybeans. Since costs of changing to narrow rows will vary from farm to farm, each farmer must decide his own best time, if ever, to change.
Shibles, R. M.; Lovely, W. G.; and Thompson, H. E.
"For corn and soybeans - Narrow Rows,"
Iowa Farm Science: Vol. 20
, Article 2.
Available at: https://lib.dr.iastate.edu/farmscience/vol20/iss9/2