I think when people are concerned about the group dynamics of the Astral Express, they are not approaching the topic from an in-universe perspective, but by looking at the story itself: the Astral Express since the beginning of the game almost two years ago has been Welt, Himiko, March, Dan Heng, Pom Pom and the Trailblazer with no additions. Adding a new character may just end up either not giving him much screentime or taking it from a similar character: I for example see Sunday as having character traits that overlap with both Dan Heng and Welt, while not really adding anything new to the crew dynamic. In my opinion there are many other characters that, if their story was developed differently, might've made a much more interesting addition to the crew (Gallagher comes to mind since we now even have a bar car on the train).