The vast amount of the surplus you are calculating is coming from the first six years of the deal, which the Braves already control for likely less money than this extension proposes to pay him.
This deal offers to pay him 25.75 AAV over seasons 7-10.
So he'll need to put 3 WAR over the balance of those seasons to earn his contract. While an MVP caliber player would certainly provide a good bit of surplus at that price point, it's entirely possible that is a fair pay or an overpay. We just don't know what happens to Acuna between now and then. He might do something to his legs that makes him fat like Matt Kemp for example.
If he's not giving you tons of surplus over years 7-10, the deal isn't really keeping any windows open. It's just keeping him on the team.
I personally would be inclined to wait and see what he actually turns out to be and then I'd look to see down the road if could maybe pay him more of a market rate for his remaining years of control in exchange for an option year or two at a hefty market rate.
That lets a mid-market payroll team have some flexibility in seeing what its roster is going to look like without taking on a huge risk.
If I can't get that, then I can't. I'm willing to trade him or let him go to free agency.