Yes, I believe he will have an OBP over .400. If not every year, certainly many years. As for his career, yes, I believe it will be possible.
As you stated, there have been 56 players in the history of the game. Who is to say that Arraez cannot be the 57th? He has been very close to a .400 OBP for his entire minor league career. There is only one person who can say whether or not he will continue to improve as he matures and gains experience, Arraez himself. I believe he will.
As for prospect evaluators, Arraez does not do the things they get excited about...thus, he doesn't get included in the upper echelons of their lists. All he has done, his entire minor league career, is get on base. I'll take that, especially amongst all the bombers he will be surrounded by.
There is a difference between possible and likely. Anything is possible, but Arraez being top 57 all time in OBP is incredibly far from likely.
If it is likely, he's (IMO) the best prospect of all time, a near lock hall of Famer, and shouldn't be traded for anyone in baseball.