Why "new adult" and "grown-up YA" literature aren't a distinct genre
There is not a NA market. There is an adult market, and there is a YA market. There is nothing in between them.
I'm not entirely sure why there is such stigma attached to these books becoming part of the adult market. I think it's a huge and welcome addition to the adult fictional landscape, especially since there are books out there currently and books that have been around for quite a while that tap into the 19-30ish world and voice. These books tackle adult challenges and do so with an adult perspective. Being an adult is such a long, varied, and challenging period of time, and it's one that is so different for every single individual. Whereas YA novels have some unifying themes to them -- growing up, discovering who you are, earning an education, dealing with adults who still exert control over you in a myriad of ways -- adult novels do not. This is why there are so many genres and why there are so many appeal factors. One of those is going to be voice, and one of them is going to be age/experience/perspective of the main characters. Books featuring emerging adults? They have appeal factors for many readers who identify with this period of life. The same can be said about books that tackle middle age romances, books that explore what the end of life might be like, and so forth. But those books aren't categorized as "middle adult" or "geriatric fiction."
They're adult fiction.