Killing Max would have rendered her entire plot line throughout this season (which is a metaphor for dealing with trauma and suicidal thoughts) pointless. And make it end on an incredibly dark note, with max ultimately failing to overcome the living incarnation of those things (Vecna) and then dying. If you really think the show needs to kill main characters to stay interesting, let it be literally anyone but the character who explicitly had an entire episode dedicated to her fighting off Vecnas influence.

Eddie basically had to die, because there was no salvaging the fact that all of Hawkins was now fully convinced he was a satanic murderer. It honesty would have been worse for Eddie if he lived, because he would have had to either spend the rest of his life in prison, or on the run. Him getting to die on his own terms, fighting his cowardice and standing against a horde of monsters, is way better for the character given the circumstances.

And frankly I just flat out disagree with you saying shows need to kill major characters to be interesting but I guess that’s just a matter of taste in media. I enjoy plenty of things where I know no one important will die.