Let's bear in mind that the author is a guy who went to several different specialists across disciplines. I'm don't think he's talking about a lack of motivation - I think it's more about trying to continue to be active in a healthy way.
The author's incentive for going to the gym was so that he could go climb or surf or ski or whatever and be both "fit" and injury-free. It's a critical distinction. There was probably a time when all he had to do was go out and make those activities a part of his existence, but I would wager that as he has aged and had different responsibilities come up just making that fitness a side effect wasn't possible. Instead, he probably tried to go ski X times a year, but got hurt the first time, and the subsequent trips never happened. He probably tried to go climbing weekly, but something tweaked and suddenly he was only climbing monthly. When it's harder to go out and run 5 miles like you used to, you're less inclined to go run 5 miles, and might slip to 3. When 3 is eventually harder than before, you might just not run at all.
I think the article is speaking more to concerns as you age and you can no longer get away as easily with just participating in the sport. Rob Shaul and his mtnathlete.com and militaryathlete.com sites address those problems quite directly, and unfortunately it involves a lot of painful work that is mostly gym based. He's designing the mtnathlete workouts to help skiiers stay healthy and be ready when the season arrives. He's training military personnel to be more durable and harder to kill. To explain, the site is a few years old, but when he started it it was as Crossfit was just starting to ramp up in popularity. Despite the advertising that it makes you better at everything athletic, Crossfit is usually all about Crossfit - there are Crossfit games now and it is in many ways its own sport now. Rob's site was very specifically geared to mountain athletes who wanted to perform their sport at a high level possible with less injury. It's not a gym program just for the sake of being a gym program.
There was a time when I could go out and make my life about doing whatever sport it may be - climbing, running, basketball, etc. - and recover quickly and easily from the activity. Now my body has aged a little, I sit in a chair too much, and suddenly a small nagging injury never goes away, and I'm less motivated to go run several miles or play basketball or climb. By incorporating durability training like the article discusses, I'm more able to avoid those prolonged periods of inactivity and keep motivation up.