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Things happen in life that change your plans: relationships, children, etc. Physically, Ichiro may be fully capable of continuing to play at a high level until 2021, but he could also decide to walk away when he feels he's left a big enough mark. Or when something else becomes more important.
By the way, what's going on with my login here? My posts are now listed as "anonymous". I thought it was something I was doing, but it appears to be on your end.
Watcher