We watched it last night - absolutely loved it!
I don't agree with any of the criticisms I've seen at all - I think it was a perfect ending. I liked Mike's story, I see it as a nice way for him to cope with the reality of El's death and also to help his friends come to terms with it (and to help the audience), which is a key part of the art of good story telling.
But I do think she died. I don't think she had fully decided until she walked out and got hit by the sonic - as soon as she realised the military were more interested in capturing her than the fact they'd saved the kids, and saw her friends being held at gunpoint, she knew Kali was right and it was the only way to end it otherwise they would have always been looking for her.
She was standing just inside the gate - the sonics were all aimed at the bottom of the ramp, which is why she could use her powers and her goodbye with Mike was heartbreakingly perfect. The line up with the lyric in Purple Rain "It's such a shame our friendship had to end" as they kissed nearly broke me!
I don't see the lack of demogorgans, demodogs and demobats as a plot hole - we don't know how many of them there were, how many were in the upside down etc - the Mindflayer and Vecna would have been expecting them to attack from the bridge, not the Abyss, so it would make sense that there wouldn't be many on the planet itself. The only thing that bugged me about the final battle was how quickly they got up on those cliffs.
I did think that there was going to be another battle with the military at the end and that ultimately K would be the 'big boss' but I'm actually glad they didn't do that.
The Duffer Brothers always said that there would be spin offs but the story for this group and Hawkins was closed and I found the 45 minutes dedicated to tying up all the story lines really good.
They've left enough open ended that they can get plenty of material, without having to keep this story line going - they could have a military spin off - K could easily carry her own series with other monsters and weirdness, we will hopefully get the prequel with the story of how the scientist came to have the Mindflayer rock in the tunnels and there are 12 children who we know have innate superpowers with Derek in particular being a very popular character, so more than enough opportunity!