So, if we climb a 5.9 and fall in a section of 5.8+ we are still safe. Again, your definition of what constitutes the total mastery of a move only applies to YOU.
Surprise and desappointed by your answer. Your question was on the meaning of lower the stress.
When people climbed on piton and hamp rope, they say: don't fall. that means that when you are doing a move, you must anticipate every possibility and choose the one with the lower exposure to danger. Today, we have better equipment, but the game could be the same and it will change how you climb. the "we" apply to many people in the last forty years of climbing. climbing a degree under is level of top rope is a current practice in many area of the world.
With bolt, you can try any thing because the fall have less consequences. So, you can explore different type of equilibrum and be more efficient to climb a route. it is less dangerous. As long as you have bolt, it is the same. But I trained every years to have better placement for my protection and you don't have to do that with bolt. So, for the majority of sport climber, it is a little more dangerous to climb with pro.
a modern definition will be good for many reason. In publicity, diversify the market by using different brand is often use. As people have more choice, they will try different option and that will be good for tourism and the industry.