It ain't a fleecing when you look at the years of control.
Hayward has a nice bat, but he's definitely a defense first guy. That's his strength. That's not a bad thing in and if itself, just interesting, because the Cards need a big bat more than they need a good glove in the OF.
Btw, extending to him means nothing in terms of this trade. They could have signed him as a free agent after this season, so the deal still is one year of Heyward for four years if Miller and a top pitching prospect. The interesting question is if the Cards extends a QO to Hayward after this season.
Overall, it seems like a fair trade for both teams. Cards fill a big hole in RF for one year, and the Braves get two young arms for lots of years.