Best leadoff hitter in the game? Someone drank deeply of the Boras cool-aid. I actually thought he was arguably in this conversation around the ASB, but he stopped getting lucky on his BABIP and his OBP took a noticeable dive.
Damon's good and he will be missed, but the impact of his departure is likely to decrease every year because he will be playing the most physically demanding position in the game one year older each time. This isn't speculation talking, this backed up by age-based trend analysis by some the smartest statistical minds in the game. PECOTA and PrOPS have him, and all players heading into their mid-30s, on a pretty steady decline over the next 4 years.
I know these systems are fairly pessimistic and are based on trends set by all players, not just he elite, but at some point these declines do occur. They did for Bernie Williams after he turned 33, just with a higher peak, but his rate of descent was actually greater than the projections. Damon's '05 was 50% higher than his PECOTA VORP, but he actually underperformed his projected WARP (5.5 actual, 7.2 PECOTA).
And his durability, a big selling point for him, is going to be affected too. He plays go for broke both in the field and on the basepaths, which is great, but his body is starting to break down from this style of play. He was a ghost of his 1st half self after the shoulder injury last year. At some point, his style of play is going to have to be curtailed, affecting his production, or it's going to put him on the DL for a good chunk of time.
Thinking his replacement will be better in 2008/2009 is silly if he doesn't suffer any drop off, but thinking he won't decline is even sillier IMO.