It depends on how a professional player sees his career. Some of them want it to end soon (for multiple reasons like injuries) and get involved in other things, while others don't want it to end and play until their 50s. Retegui will take 80 million euros if he "runs out" his contract. Money that he will never get in any other team in any other league. At his 30th year will be wealthy and free to do whatever else he thinks is best for him. Not to mention that a lot of such transfers hide other things and other moves...