Well yes we lose a bit defensively with Seiya for sure. We're gaining on offense (OBP, SLG and HR). They're gaining on defense, speed, contact, versatility.
Whether the Phills would do it is questionable, but I think it's a pretty fair trade. Schwarber has a year left on his deal. The upside on Bellinger is higher IMO. If he's healthy all year, plays mostly CF, and doesn't DH I think he's a 3+ WAR player and probably better than say Happ.