Egg donation does not diminish a woman's fertility. A woman is born with a defined number of eggs that will be lost with each cycle, becoming useful only a few, which are those that reach maturity and the rest are eliminated naturally.
In each menstrual cycle of a fertile woman only one egg matures (sometimes more), with respect to a number of oocytes (10-20) that would naturally degenerate. What is treated with an egg donation treatment is to stimulate the donor's ovaries with hormonal medication, so that more follicles grow and thus maturation occurs in a greater number of oocytes that would otherwise be lost because they would suffer atresia.
Therefore, by recovering those oocytes that would be naturally eliminated, we are not affecting the fertility of women who want to donate eggs or their ovarian reserve.