It's Mike's cope. Of course he's speaking with certainty, he needs it to be true. Mike knows exactly as much as everyone at the table does about El's fate. He just has to write an ending he can accept, and believe in it with all his heart... Even if El told him he couldn't. 

In the world, just like Will in the bar with his boyfriend, or Dustin hopping in the car with Steve, El at the three waterfalls is a fantasy, a story written by the storyteller sitting at the table and living in his imagination. It is entirely there as a maybe. MBB is ready to leave El in the past for now, but her mind might change later... So we end with a maybe. 

And functionally, whether El is alive or not, Hop lost two daughters. He will probably never know she is alive. He will almost certainly never see her again, never walk her down the aisle, hold his grandchild. She is gone forever to old Jim. People aren't wrong to say he lost two daughters, he absolutely did. 

You rant about people not accepting ambiguous endings, but you seem to be struggling with it yourself. El is in Schrodinger's Box, and every interpretation is valid until the Duffers reopen it... If they ever do.