My guess is that like lots of other medical wisdom/folklore about the adult brain, we'll eventually find workable exceptions to the idea that favoring one eye is irreversible after a certain age. Physical qualities of the eye that contributed to the initial imbalance can now be surgically corrected; neuroscience has found that creation of new brain cells continues far later in life than previously thought; some drugs (including SSRIs) have been found to trigger neurogenesis in adults, as does exercise.