"Being happy all the time gets old real quick", when it comes to media that doesn't mean having to completely play god to completely fuck with the narrative out of nowhere just to introduce conflict or have some literally last second "character development". Especially not for such an important thing like an ending. It's like if in real life someone is living their best life, but someone out there went "hey! I got bored so I gave you super cancer now! I'm sure after months or years of treatment you'll learn to love life more when you recover! teehee!"
Hell, you said that the ending was supposed to be even more brutal, but something must have stopped you from going through with that. Whether that was higher paying supporters not liking that idea, maybe someone you talked to about it advising against it, or maybe you yourself realized that doing so would be the epitome of stupidity. In the end, your mind was made up to follow through with something so undesirable even if it had to be toned down.
If this ending is truly the way you lovingly wish to end the piece of creative work you spent 4 years of your life on, and it left you yourself satisfied, then good for you. In the same way you spent those 4 years working on this, a lot of people on here followed and enjoyed your work for that long as well. Some from the beginning, others just recently. So despite this being a game people get to graciously experience for free, it doesn't mean their voices and opinions should be invalidated. You can see for yourself how many people were left completely upset by this ending. For many, the ending is truly a big stain on an otherwise great game and a blight that will inevitably remain with you as dev moving forward.
At the end of the day, it's not that deep, life goes on. Everyone has creative liberty, so if this is not quite your perfect ending but a satisfactory enough ending for a work I'd assume you still care about; then all we the players can do is accept that, since at this point it seems like you are dead set on leaving things unchanged. You have that right.
Good luck on your next game, may it serve as a good distraction towards the backlash because I can't imagine it being easy seeing such criticism by so many.