Huh? Free agent? How is the comparable? The beauty of draft picks is that you can get players far cheaper than in FA.
It's not like draftees are signed for life. If they were, I'd see the point a lot more. But a third rounder is only going to have, what, a four-year deal anyway? So he'll be a free agent at that point and all those remaining years don't matter, unless you re-sign him.
Now if this were a first rounder with a seven year contract and you thought a guy would be rapidly declining by 33, I'd see the point a lot more.
Finally, I'd argue that Witten is a lot "older" than his chronological age would indicate. In a lot of cases it's not the years, it's the mileage.