Perhaps the brain is simply making use of extraneous matter for energy or its purging out unnecessary neurons. Maybe it decreases creativity, but increases arithmetic ability.
My entire point is we have no idea what the end results are, just because there's a reduction doesn't necessarily indicate a problem.
Myelin is pretty damn important for the brain's function. Signals go much, MUCH slower without myelin, and there are a number of clinical disease and problems that can happen thereof. One of the main effects of B12 deficiency is loss of the myelin sheath around the optic nerve causing blindness, and the psychiatric effects of losing myelin in the brain -- in addition there are a lot of disorders where a primary effect is myelin degradation / loss. It's been very well studied.