I had a little fun with this one, but before you bash me for being too optimistic, consider this.
I have been wanting to do a fun "guy lasts forever" Crystal Ball for awhile, but I had to pick the right guy. I think Markakis fits because of his combination of good overall athleticism, success at a young age, and excellent work ethic and makeup. If he avoids serious injuries, he is the kind of player who should age well.
Remember what these Crystal Balls are for: to stimulate discussion. With that in mind, what do you guys think about this CB? It shows Markakis' peak at 31 homers. Is that too many? Too few? It shows him topping .300 just twice in a full season. Is that too few? Too many? Does he have the kind of offensive skill set that would make him a candidate to be an effective pinch-hitter late in his career?
Note the progression in his career....emerges as a solid guy quickly, career year at age 28 (in which he is traded during the stretch run), then he scuffles at times after signing with the Mets as a free agent. He returns to Baltimore, does well for a couple of years but starts to slip in his mid-30s, ends up in a platoon role, then has a late-career renaissance as a pinch hitter. This may not be what happens, of course, but it is a logical path.
He could also blow out his knee at age 26 and be finished by 30.