There are a number of posters that seem to be misconstruing what the "law of diminishing returns" actually means.
The term is used to refer to a point at which the level of benefits gained is less than the amount invested, or in the case of heavier heads, the positives of using heavier heads aren't equal to the negatives.
Yes, you can keep adding weight to the tip of an arrow to make it weaker, but is it worth what you are giving up in terms of speed, trajectory, and penetration. It is a two sided calculation.