Yeah they just lost motivation. Dude my original reply agreed with you, it's why I was so confused when you started arguing about it.
Players like JaCkz and roeJ prove that when older players get the chance to be a top player they can still be amazing players. It's just that players who have played the game for 10+ years who reach 30 start to lose motivation because of how long they've been playing. Those are the main players in the scene who people have seen reach that age, so it's not too outlandish for people to believe that you just get worse as you head towards 30. In reality though, it's just motivation issues.