If it had been made clear he was available, I think teams would have been offering a lot for him. Can't imagine the Cardinals wouldn't have put together a substantially better package than they did for Heyward.
I'd imagine that some other teams would have kicked the tires as well. At least the Orioles and Cubs, Royals could have been an interesting dark horse with a lot of young talent they could swap. I think Detroit would love to have him, but they probably don't have the young players and prospects to work out a trade. Yankees could have been another dark horse, looking for a new face of their franchise with Jeter being gone. Certainly they would have the money to keep him around.
All of that said, I'm not sure it's a baseball decision, which is what winner77 doesn't seem to understand. His operating assumption was this: "If they had traded him and gotten a big haul in return, it would increase their chances of winning, which always makes fans happy. " I don't think that's an accurate assessment of the situation in Miami. Winning hasn't brought fans in; they've won 2 WS, and nobody came. They need to attempt a new tactic. Maybe keeping some players of Stanton's caliber will give people in Miami something to actually connect to. The turnover on that team has always been incredible; that could make it harder for people who aren't already Miami baseball fans to build any kind of connection to the team. From a baseball perspective, in Miami's financial situation, a contract like this is a massive handcuff, and it's going to be tough to build a winner around. But at the end of the day, the team is trying to make money first and foremost; winning games is secondary to that goal. I think this is more an attempt at the former, willfully ignoring the latter since focusing on that aspect has worked so poorly for them in the past.