The problem is Stanton is the guy most likely to generate any actual income for your team. He's a league MVP, HR champ and gate draw.
Meanwhile the Marlins are still paying Brad Ziegler ($7mill) and Junichi Tazawa ($5mill), and will pay yhe arbitration costs for Marcell Ozuna ($11mill projected) and Dan Straily ($4.6mill projected). Unloading those players, all of whom are tradable for something better than an A ball pitcher0, saves more money per year. Short term but you keep your best asset. and if Wei-Yin Chen ($54mill per 3 years) comes back healthy, he becomes a trade candidate.
And don't forget the reason Stanton got that contract in the first place was to show Marlins' fans this team wasn't always going to be about firesales and salary dumps Stanton received that offer in the wake of the Josh Johnson/Jose Reyes trade with the Blue Jays as a how of good intentions to the fans.
This trade just pounded the biggest nail into the Marlins' fanbase coffin. The man who was meant to be their symbol against firesale salary dumps was just dumped somewhere else for nothing.
It's a shame they worked so hard to make MLB pay for that big new stadium a few years ago. Maybe they can get some cash back there selling the chairs. They won't be needing them because whatever fans were left are not coming back this time...
But.hey, they saved a few bucks doing it.