They'll be parked when its time for a Heavy Maintenance Visit. 757s never get to a fifth HMV as the aircraft is well beyond its useful life. Hell, four HMVs for the plane is impressive unto itself. The additional 21 757s Delta decided to keep, in addition to the 56 1995-later builds, all have a good 5-6 years left before they hit the scrapper. I expect the four 1987/1988 build 757s to be parked this year. I give Delta a lot of credit finding a way to make the 757 profitable with a dense 20F/179Y configuration, making it possible for the aircraft to be replaced by a far more efficient aircraft next decade, whether it be the 737-10 MAX or A321 NEO.