The problem is in 5 years he'll be another Michael Morse on defense. He gets poor jumps as it is, age isn't going help him. As it stands now, he's nearing the apex of his defensive abilities while playing a non-critical position.
Michael Morse was the slowest guy I've ever seen play for the Nats. Harper has above average speed both as a base runner and an outfielder. This article makes a pretty good case that Harper is likely to return to being an above average right fielder and that last year was probably more a fluke of the small sample sizes of defensive stats.
https://ftw.usatoday.com/2018/11/bryce-harper-defense-uzr-drs-mlb-free-agency-2018
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