Problem with the "everyone would just be happier with a harem" argument is that it inherently assumes that none of the female characters could be happy ending up with anyone besides the MC. Which is dumb. A lot of shounen/seinen romance stories are written so that the MC is the only single, non-horrible, non-rapey person in the whole universe but of course reality doesn't work that way.
Sure there might be some heartbreak in the short term but that doesn't mean they can't get over it. But writing that in would requiring acknowledging that the MC might have some actual competition out there, which terrifies the audience who want to self-insert so that's no good...