I believe that while initial firm orders are for 14 (Delta people, please confirm - I can never keep up with these!). The total commitment, including options, may well be as high as 60. So, yes, the 777 will be a part of Delta's fleet for many years to come . . .
And while they won't be rushing anything, I would imagine that the 777's arrival will signal the beginning of the end of the MD11.