2) It still has popularity
When you look at the big picture of the One Chicago franchise, Chicago Fire still has a very large and vocal fan base. Some of the most popular characters in the whole fandom, such as Casey and Dawson, come from Firehouse 51. NBC axing the show would also come with the risk of alienating a large part of the franchise’s audience.
Part of that comes from the fact that again, Fire came first so we met these characters first and have known them longer. But there’s no denying that it’s also the favorite show of a number of One Chicago fans. And while a large portion of the audience also watches Chicago PD as well as Chicago Med, it’d be a mistake to assume that every fan is tuned into all three shows and that those Fire viewers would just content themselves with the other two series. If Fire doesn’t come back, there’s going to be at least a small section of viewers that tune out.
And we need the show to keep those characters. Of all the One Chicago shows, it’d be hardest to work the Fire characters into cameo appearances on the other two series. Cops and doctors can be moved around fairly easily, but firefighters need a fire. So without Chicago Fire, we’d have to have either Med or PD coming up with fire-related stories if we ever wanted to see Dawson, Casey or Severide et al. going forward. It’s just good logistics to have Chicago Fire season 7.