As mentioned above, Severide has fallen off the map in the past two seasons of Chicago Fire. Meanwhile, Stella has stepped up to the plate and been a steady presence in the NBC procedural drama.
Stella has put in the work, and consequently, she deserves to be the representative of the first show in One Chicago’s shared universe. If Severide had been around more prior to Boden’s exit from Firehouse 51, it would make sense if he was the face of the show. But that wasn’t the case, and we should expect Stella to have a bigger role when Chicago Fire season 13 premieres.