Ender is roughly a league average hitter whose defense has already entered its decline phase. He is currently a 3.5 win player, and that value will probably level off and start declining.
That means he could be projected for ~13 wins over the next 5 seasons at a cost of $36M. Surplus value in the $90M-$100M range.
Happ for Inciarte is the type of trade the Braves would have to make if they want to avoid pushing the rebuild back another couple years. However, that all assumes Acuna is adequate in CF, and I'm not sure where that notion came from.
If Mallex were still in the organization, trading Ender for a player at another position of need would have made some sense. In fact, it's a move I suggested. Now that Mallex was traded for even more pitching, trading Ender isn't really an option any longer.