It is worth looking at what the market for Stanton might look like. I've seen assertions that the fact no one claimed him off waivers means he can be had for free. I doubt that is the case. Teams have more of an ability to adjust their roster/payrolls in the off-season and the market for his services will be very different. I'm willing to bet he either does not get traded this off-season or if he does the terms of the deal will make clear the contract was not all albatross (the Marlins receive positive value in return).
Any analysis has to start with his expected production going forward. Stanton is on his way to his 2nd 6+ WAR season in the past four years. This is his age 27 season. Lets first look at expected production over the next three years (which cover the years ahead of his opt out clause which kicks in after 2020). For his age 28-30 seasons, I think 5 wins per year is a reasonable projection. Total of 15 wins for those three years. There is both some upside and downside risk relative to that. His contract calls for him to be paid 77 million over the next three years. I expect the market rate for free agents this off-season to be about 9 million per win. The 77 million Stanton is going to be paid will get you about 8.5 wins this off-season. So Stanton's expected surplus value is about 6.5 wins (15 minus 8.5) for the next three years.
The tricky part is assessing what comes after the opt out clause kicks in. I think the odds are high Stanton will opt out. The post-opt out part of his contract guarantees him 186M for his age 31-37 seasons. Over those 7 seasons, a reasonable (conservative in my mind) estimate is he will average 3 wins per season (for a total of 21). A conservative estimate of salary inflation would put the going free agent market price for a win at about 10M in the 2020 off-season. In other words, teams would play a player like Stanton (age 30 and projected to produce 3 wins per year) about 210M on a seven year deal.
There is a chance something bad (productivity curtailing injury) will happen and he will not opt out. This is the most difficult part to evaluate and I'll do it the quick and dirty way. I would put the odds of a no opt out scenario about about 10%. Let's say that a potential trade partner with the Marlins estimates that they would lose about 10 wins of surplus value in this "bad" scenario. Assuming about a 10% chance of this bad scenario would result in a downward adjustment of bid by 1 win (10% chance of a negative 10 wins scenario). But keep in mind this must be weighed against other outcomes in which his expect value the next three years is 6-7 wins. This is why I'm convinced Stanton's contract is far from being under water. If a trade happens this off-season, the other team will be sending asset(s) of significant value in exchange for him.
Ordinarily, I would agree with your reasoning.
However, the Marlins are a wounded animal. They have publicly said that they will get the payroll down to about $75M. They have several contracts that are just unmovable (Chen, Volquez). Then they have several contracts that are unlikely to be moved (Prado, Tazawa, Ziegler). Cots showed them at $115M for 2017 with Chen increasing by $1M, Stanton by $10.5M, Prado $2M, Volquez $4M, Gordon $3M, Ziegler $2M, Tazawa $2M, Ozuna was $3.5M in 2017 and will be arbitration through Boras so estimate he increases by $5M, Yelich $4.5M. So, an increase of $32.5M just with players under contract coming into play the the changing of the calendar from 2017 to 2018.
The only contracts of significance that they can lose just by not renewing or allowing players to leave via FA are: Ellis $2.5M, Ichiro $2M (has $2M option for 2018), McGowan $1.75M, Dietrich $1.75M. So, they could shed about $6M in contracts ($8M - $2M in league minimum replacements).
So, if Florida does
nothing then their payroll goes to about $142M just with the changing of the calendar.
To get to $75M they will have to do a serious tear down and find a way to move some assets that normally
can't be moved.
That's why I think it is irrelevant as to what Stanton's future value is. The Marlins, if they follow through and try to go to $75M, will have
no choice but to attach value to contracts with negative value to get rid of them. Any team willing to help them with that will be at an advantage of getting some good players without having to give up talent value going the other way.
So, if you want Stanton? Then you have to take Ziegler.
You want Ozuna then you have to take Tazawa.
You want Yelich you have to take Prado and Volquez. etc. etc.