Well, look at how many ski racers, and competitive snowboarders, both racers and freestylers have many, many more problems than just worn knees. If you look at the statistics, competitive weightlifters and powerlifters have far, far fewer injuries than people in snowsports. I don't think I know a racer who hasn't had multiple ligament tears and multiple broken bones. They are on a first name basis with their ortho doctors. I don't know any world class competitive ski racers or snowboarders in their mid 40's, but many of the world records in powerlifting are held by men in their 40's. People compete in weightlifting regionally well into their 70's.
As far as snowboarding being an endurance sport sport I have to disagree. An endurance sport is like running a marathon when you don't stop for 26 miles (or whatever they run). Cross country skiing is an endurance type of sport.
Downhill skiing and snowboarding is a power sport. You ride the lift up, make fast, explosive turns down the hill maybe for 1 or 2 minutes, then rest while you are riding the lift back up. Sure, you might make multiple cycles of this over the course of a day, buy you are not riding constantly all day. That is the definition of a power sport. Cycles of fast movements followed by periods of rest.
Every national ski and snowboard team trains during the off season for power and balance. They do olympic lifting, sprinting, and balance work. They do very little long distance, endurance type of work.
Of course, we need to separate the needs of competitive snowboarders and recreational riders. If you approach the sport purely from a recreational perspective, then yes, any type of dryland training will be better than being a couch potato all summer.
Considering how non active the average person is now, everybody on this thread should feel good about themselves for both snowboarding and doing some type of off season training!:)