What was the main reason behind season fives ending

It seemed to me the shows pace was really slow and there was many plot points but now that im watching season 5. its rushed. there are characters entering for no apparent reason or build up. the emotional talks are accelerated even fighting in the upside down within the team that was needless. i dont fully recall but i remember that kid in season 4 finale being major thing as he was dying in there, but the first episode in 5 he just gets scooped up and isnt referenced again. the poison spores in the upside down not affecting anyone. there is alot of things that didn't make sense. I was hoping those with more information could clarify what was going on.