If I were a professional, I most likely would be going through enough yeast on a regular basis such that its degradation with time and exposure to atmospheric oxygen and moisture would not be an issue. However, it is true that an opened packet or bag of IDY or ADY will lose some of its leavening power as compared with a fresh supply, even if stored in an airtight container and kept in the freezer. However, modern yeast strains have been developed to tolerate longer storage periods and temperature. In my case, I store my ADY and IDY in airtight containers that are held in my freezer compartment. Because of loss of leavening power with age, my practice is to increase a measured amount of the yeast by a small amount. In the past, I have had bags of IDY that have lasted for several years. It would take specialized laboratory equipment to be able to measure the degree of degradation of the performance of yeast with age and, without such equipment, I am not sure that I can perform a simple and reliable side-by-side experiment in a home kitchen environment to be able to measure the differences in leavening power. But there is no doubt that the optimum performance will be achieved using a fresh supply of ADY or IDY. All of these issues are inherently moving targets. For example, the same yeast in a cold climate or an excessively humid environment will perform differently in a warm climate or a dry environment. Also, different people measure out yeast differently, often with different shapes and designs of measuring spoons, and some use different quantitative standards, like scant, level and heaping. I personally use level measuring spoons. Others may use special scales that can weigh small amounts of lightweight ingredients like yeast.