Duffer Brothers say Eleven had to go so the other characters could move on.
"There was never a version of the story where Eleven was hanging out with the gang at the end. For our characters to move on and for the story of Hawkins and the Upside Down to come to a close, Eleven had to go away."
end credits like these are so important because regardless of if you loved or hated whatever you saw before, you’re reminded of the beautiful roller coaster of a journey you go on while watching film & tv.
one of the best end credits in recent memory.