"Not all players" is a bit of an understatement. 
When I say the "same number" of players age 33 or over hit 3.5 WAR, that's only 4 players total, and 3 of those guys (Zobrist, Turner, and Lowrie) are hardly guy you would have said were franchise players at Bryce's age.
There is really very little chance that Bryce Harper is a productive player past the first 5 or so years of his deal, as I've pointed out in other posts looking at aging curves. Although he does share some traits with Joey Votto, who is the lone "franchise-type" player aging gracefully into his mid-30s.
Harper is 26. By the time his first 5 years are up, he will be 31. There have been over 100 players who have posted at least 3.5 WAR from ages 31 to 33 in the last ten seasons.
With modern workout and diet regimes, it a naive to think someone like Harper cant be good after the age of 31. Beltre, Votto, ARod, Matt Holliday, Cano, Utley, Kinsler, and Bautisa all put together quality seasons at ages 32-35. You should expect Harper to do the same, which means you're going to get 7-8 years of good-to-great plate performance from him.
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