If the need for a certain amount of mass is a "myth", why not use a plastic hammer? Why don't baseball players use a feather instead of a bat?
Firing a primer requires momentum - mass times velocity. Factories balance that formula to achieve reliability, but there is a reason most guns with very light hammers have strong hammer springs. If one side of the formula is lowered, the other has to be raised. If both sides are lowered (bobbed hammer plus weakened spring), the result will be lowered reliability.
There is enough built-in leeway to allow some playing around. But it is just plain silly to take the view that hammer mass doesn't matter and reduced spring force doesn't matter and that reliability will never be affected no matter what is done to either or both.