By offering full benefit senior pass at discount, you're hoping to capture the family of those seniors.
I am skeptical that many people make decisions on where to ski based on whether grandma and grandpa can get a cheap pass. I am skeptical that even grandma and grandpa are making decisions on where to ski based on the availability of such passes. By the time most people are eligible for these discounts, they already have strong preferences on where they ski based on other factors. If I'm going to Killington for 50 or 60 years, have property and a ton of friends there, I'm not going to switch to Sugarbush because I can get a cheaper pass. It's the same as cdskier's rationale for why Vail shouldn't worry about disavowing Hunter's lifetime passes.
My guess is that senior discounts kind of just happened at resorts because people are familiar with them from other contexts where senior discounts were intended to relieve financial burdens on people on fixed incomes who do not make significant demands on services (like senior discounts for bus and subway fare); as resort operators become larger, more sophisticated and focused on maximizing profits, they realize that they don't make economic sense and eliminate them.