Why do people say this? He doesn't have a history with Athletico. He wasn't a Madrid boy. Hell he is not even Spanish. He was already good before they bought him. Sure he got better (or performed better with better talent around). And if we check the history of Athletico, they've sold every single top striker theyve unearthed the last 7 years.
So nothing factually, historically, or emotionally supports the above claim. Do I think he is leaving? No. But if a good offer was made, history suggests Athletico will sells. Why? They believe they can replace anyone with their excellent scouting and recruitment. So far, it's hard to doubt them as they've done it 5 times in a row.