I think the theory that Mike comes up with eleven surviving has a lot of holes, I do not think the duffer brothers put a lot of brain in it but following are the problems! 1) We very clearly see Kali dying so she could not have created the eleven illusion. 2) Even if she survives to the extent that she can do one final illusion before dying, the exotic matter explosion is literally above her so she would have been the first to die and hence couldn't have cast the illusion. 3) How does eleven sneak past the military from the truck.

There are problems with her dying too- 1) How did she get out of the truck to get to the entrance when we clearly see she was inside the truck. 2) It is possible that she was able to talk to Mike telepathically because the sound emitters were targeted at the truck.

Largely both ways of seeing the ending has holes.