SI.com's Cliff Corcoran notes that plenty of great hitters have put on weight as they got older; almost every player (and non-player) does. And many of those great hitters just kept on hitting, from Willie Stargell and Harmon Killebrew to Frank Thomas and Jim Thome. But Prince Fielder, Corcoran notes, is different:
Those all-time greats had room to add weight as they aged. Fielder, like the group that included his father, does not. As a result of that difference, when Fielder begins to add weight in his thirties, as most athletes (and non-athletes) do, it could slow him down to such a degree that he's simply unable to compete at the major league level. That is the trend for players his size. They don't have declines, they just vanish because there's no room for them to get bigger or slower and continue to compete at the highest level. The gap between being a star player and out of baseball is tiny for players like Fielder compared to more athletic players who can age more gracefully.
I don't have a great deal to add, except that we're in uncharted territory here. And that I would like to have been a fly on the wall when Dave Dombrowski was trying to convince his owner that nine years is a really, really, really long time.