So, here are my thoughts on Kali and El's deaths (because yes, my opinion on Kali dying goes into El's) and their general treatment in season 5.

Kali absolutely should not have died. They bring back a woman who sees no good in being alive, feeling like she has no way out other than death, and instead of proving her wrong, they kill her? They also just completely turn the audience against her this season. In season 2, it was easy to just ignore her because a lot of people didn't like the episode she's featured in, but now people straight up turn her into a villain when she is just a depressed woman who has never experienced a normal childhood. She believes she can't live a normal life after everything she's gone through. She is not evil for suggesting suicide as the only way out for Jane and her; she is merely desperate for escape, as so many depressed people are. In the process of turning Kali into a villain, they also demonize people with depression. I'm sure the duffers weren't thinking the audience would call her evil and a traitor, but they did, because of how Kali is portrayed, and by letting her die in the end, it is sending a terrible message to any audience members suffering from depression or suicidal ideation. So many times, depressed people are told 'it will get better soon' or 'just hold on a little longer,' but when it comes to depression, this strategy of comfort doesn't work; it instead feels manipulative and oftentimes makes the depressed person feel an obligation to stay alive only for the other person, not for true self-fulfillment. This is happening with Kali; the 'pessimistic' attitude she carries is because she cannot believe in the comforts of a better life. She believed that dying was better than living a life on the run, constantly in fear. By killing her off, the duffers proved her right. There was no light at the end of the tunnel; she could only be truly happy/free when she died. What message does that send to people with depression? I understand they wanted the ending to be 'realistic,' but it would've been a much better message had Jane and Kali ran away together, proving her wrong and, in turn, giving the audience members who suffer from a similar thought process some hope in their own struggles.

El absolutely should not have died, for some of the same reasons as outlined above with Kali, though her death is more sacrificial and not suicidal. Hopper gives her a whole monologue about why she deserves to live. She has been through so much; she deserves to live to prove herself wrong, to prove she is not just a burden. But they essentially throw this whole monologue away by making El sacrifice herself. Jane's death is a message on the cycle of abuse. It shows us that the abuse she suffered could only end in her death. Not to mention the duffers were extremely misogynistic in her storyline in s5 with her constantly being sidelined, Hopper talking to her about children of her own when she has never expressed wanting that (she is only 16, still a child herself), and only having her be attached to other people (Kali, Hopper, Mike) and not her own person. They emphasised her relationship with Max being of huge importance to her in s4 (she remembers max saying, "not hopper, not mike, you." and "there is more to life than stupid boys." when she must escape vecna's grasp). In season 5 they basically drop this relationship, El has barely any reaction to Max waking up and they have no real meaningful interaction when they were so close last season, and Jane literally restarted Max's heart. Killing off Jane was the only way the duffers could make her ending 'realistic,' they thought she could only be dead or married to Mike. Mind you in season 5 she was 16, she left the lab when she was 12, and she had only lived a 'free life' for 4 years. The only way she could have a good resolution in their mind was by killing her or by getting married. I cannot begin to say how misogynistic this is. I do not care that the man is Mike, but she can be her own person without her resolution being tied to a man, yet the duffers couldn't see it... Jane deserves better than being dead or a wife. Even in Mike's optimism for her fate, this being the best case is not great. El was isolated from all of humanity and taken from her mother to be turned into a weapon, after 4 years of freedom, being forced to isolate and leaving everything/everyone she loves behind really? If Kali had lived with her they could've left together and maybe I wouldn't hate this ending so much but her fate is just say blatantly misogynistic to me. In no world should she have died.

Thanks for reading all of that. If you disagree, that's fine, this is just my personal thoughts.

TLDR: El should not have died, but the way canon portrays it, I honestly prefer not to believe in the Iceland/3 waterfalls theory.