How about: Developers might not want to work to the full extent of their capabilities all the time (especially as they get older) because it has additional cost

How about: Developers might not want to work to the full extent of their capabilities all the time (especially as they get older) because it has additional costs to them (stress, burnout)? The analogy is a bit stretched, but imo still works even when talking about the consequences (which often easily translate into cost) of owning Ferrari/working at high paying job.