Fifth Harmony is a popular band. Usually, in the course of a band's performance history, there are performers that come and go. This is true for all of the most famous bands over time. Camila Cabello is a great musician, but she is just one member of the band. The band will go on without her.
Camila Cabello's decision to leave Fifth Harmony could very well have a negative impact on the band. Many times certain members of a group will leave to pursue other avenues. Whether or not the band can go on, will usually depend on the position the person held in the group. As Camila was singer and song writer for the band, it may make it difficult for the band to carry on.
It's happened before and it will keep on happening, when a band member leaves the band disintegrates. I suppose Fifth Harmony will keep on playing but it's more likely that Cabello will carve out a successful solo career. The only reason she left, no matter how much she denies it, is that she is better and the band was holding her back.
Girl groups rarely seem to survive the exit of group members. Frankly, history is not on their side. It also doesn't help that the group was artifically created as a part of reality television, something that has not helped to promote band cohesion with other groups. Though there music sells well, they are pedestrian in their styling, failing to truly stand out in the music industry. Thus, without a new member (or album) of sufficient "WOW" factor, the group will quickly fade from public consciousness.