Assume there's a 100% chance he only opts out if he is absolutely sure a team is going to pay him substantially more than $218 million. Much like A-Rod opting out of the first mega-deal...
you would think teams would look at what happened with the arod deal and learn- before a player's age 25 season, a mega deal may work out for all parties, after the age 30 season, that becomes more questionable. Right now, Stanton's getting paid $29M for his age 31 season, $29M for age 32, $32M for age 33, $32M for age 34, $32M for age 35, $29M for age 36, $25M for age 37, $25M for age 38. Maybe with inflation, he beats that, but that's a lot of money for decline years (to make it worth his while, some team will either have to tack on a bunch of years of a bunch of AAV in the same number of years, either way, it's a huge risk for that team and for Stanton to opt out). Similarly, ARod opted out after his age 31 season (his original deal had two years left, each of which he was extremely good) and resigned tacking an additional 7 years onto his deal carrying him through his age 41 season; I wonder if they regret that now? But yes, if he's absolutely sure, he'll opt out, but there is no way he can be absolutely sure, so why risk it?
