It's a bit early to say that Stanton is completely finished as a player.
He's clearly not elite anymore. More than that, he's not all-star level anymore either.
Tremont is probably right.
When Jasson Dominguez returns, his bat has a much higher upside (provided he shows improvement as a young player)
At that point, Stanton becomes more of a JD Martinez type player -- a guy who probably isn't worth keeping in the lineup except against left-handed pitching.
Edit - It is worth noting that Stanton has been a great clubhouse guy and teammate and I think he's a total professional. Who knows how he will handle his eventual demotion from full-time play. But I am a fan of him.