If you worked some of those additional years between 57 and 67 and if your income during any of those years was greater (on an inflation-adjusted basis) than any of your existing 35 years of earnings, then your SS benefit would go up. But probably not by much, especially if you were a relatively high earner.
Your existing SS benefit won't decay or anything. There are some wage and inflation adjustments that will be applied between now and whenever you apply for SS, but those are beyond my understanding.
If it helps, I've been retired for about three years and check my SS benefit about every year. So far it hasn't gone down at all. I think it may have even gone up a few bucks.