This has been discussed hundreds of times, but the reasoning never changes.
Senior pass allows fixed-budget retirees to camp more often, especially in the off-seasons when the parks are not full. Due to that financial fact, seniors spend more money annually in the parks and in the out-of-park local service areas, like shopping, fuel, other stuff. I'd bet most seniors do not enter or camp during the summers or busy seasons, and least we don't because of the crowds and traffic - and because we can go other times.
That's beside the fact seniors paid taxes for fifty years or more building the parks and protecting the wilderness. Seniors have earned a little respect, and the discount. Same issue with veterans.
If you disagree with the discount policy, don't use it. BUT, leave the discount in place for those seniors less fortunate.