Is SharePoint replacing iSites or the network drive?

Not necessarily, and may actually complement these other tools. It may be that you find certain tasks easier or more convenient in SharePoint, and so decide to use it for those areas. Either way, you will decide which Harvard tool works best for each of your administrative and collaboration needs, and you are free to use that tool as needed. Your department may have specific recommendations or requirements to utilize SharePoint for a particular purpose or project, but SharePoint is not mandated in any way.