The ending is something that, the more I sat with it, the more I liked it.
I'm no writer, so maybe my opinion isn't worth considering, but I felt that it was conveyed that she suspected that she would die when killing Aku. Knowing this, she still went through with the plan, and in dying, she restored the world to the beauty and wonder that Jack once knew and felt he had lost forever, as her gift to him. In that sense, I found it to be unbelievably romantic.
An area that SJ seems to exceed in is its visuals, and I found the scene at the end under the tree to be really beautiful.
I don't know. Maybe that's just me being a romance-starved lonely sad sack of shit.