I think it would be ironic if a team with a purported glut of SS prospects would have to go outside the organization for a SS. The problem is a lack of 3rd base talent. Without that hole maybe Sano would already be a first baseman. Does Arraez's bat or arm play at 3rd? I don't think that's the best fit. I previously suggested the possibility of Schoop at 3rd. Would it be frightening to have Schoop learning 3rd while Sano learns 1st? I don't want to find out. There is no question they have the money to sign a third baseman, so pick up one of the top three mentioned above and move Sano to 1st. Gives you a year with Cruz at DH to see if Sano has a future at 1st. If they can't get that deal done they might have to settle for bringing Cron back who I thought had a nice glove there, except of course on Arraez's crappy throw.
We may have had a glut of "SS" prospects but none of them turned out to be a plus defender there. Royce could get better there, but he's not very close to being ready for the majors.
If I was in charge, I couldn't stomach another year of Polanco at SS and Arraez at 2B. Arraez doesn't have the traditonal power of a third baseman, but hitting .320+ makes up for that. His arm there isn't ideal, but I think that's probably comes into play less than we think. His range would at least be ok there.