"It's more likely that there is some trade-off which we have not discovered yet."

Clearly some tradeoffs are less relevant today than they were to a tribe of cavemen. Even if grandpa is in great shape, he will have a hard time hunting thay wooley mamoth, and having him around too long will be too many mouths to feed.

Also it's hypothetically possible to give everyone a ripped physique without gym memberships, downside of this would be increased protein consumption.