So I'm on the last chapter of TLAT (again) (after so much delay too) and I had to remind myself that in real life, a tragedy of that magnitude has NOT occurred.
And so while I did that, I realised why the story needs an end: It has no hope. I realise that this is a bit redundant as most of the people we read about have very tragic lives, but it's not really true is it? Their ends are yes very tragic, but they had happy times and we deserve to see that. And honestly, it all turned out for the better and made a difference in spite (or more probably, because) of the terrible loss incurred.
We, as an audience, deserve our happy ending, even if it's not actually that happy. We deserve it cause it's tries to be happy.
Lily said it herself (to James after the whole sixth year drama), we need to come back and attempt for a better year because why would we leave it somewhere so sad. The last chapter is our hope for them to be happy. Fuck JK and her sad story, we can still hope in their present and allow them to use that sadness and fuel it to a future where they don't have to be in fear anymore.
Anyway that's my two cents. I'm an optimist at heart and I just want an ending.