I am, at this point, assuming the Phillies would have signed Arrieta (or even one of the other guys) regardless, though we'll never know how much "well, we already gave up the 2nd-rounder" factored into their willingness to sign him. Similarly, they might have signed a pitcher first and still decided to get a bat for the same reason.
This is very much of-the-moment thinking but since it is the draft and the Phillies look like a WC team at best and the core first and second-year players look like they may need more than one season to truly develop, I do wonder if giving up two picks for two short-termers when the team may never draft this high again was correct. That said, they can probably replace that lost depth when and if they move a 2B or one of the OFs. Or heck, if they traded Santana at the 2020 deadline.