Responses
We don't hate the couple, but it just doesn't seem right to most readers. Some feel unsatisfied with the outcome of the story because we all know Kei was using Akane as a rebound but he eventually fell in love with Akane though I still wonder if he really does love Akane or he just loves the feeling of being loved and taken care of by Akane that looks exactly like Mei. So yeah I think that's why people are whining about the ending cause it seems a bit forced and rushed.
Everyone seems to be hating on the couple. Grief can bring people together. They find comfort in each other and have shared memories of someone they both loved. As he said, they are starting again, learning to be more with each other and not just basing the relationship off Mei. Quite frankly I think that is what Mei would have wanted, both of them to find happiness.