A PBX, even in its cloud-based version, is primarily focused on voice and telephony management: inbound and outbound calls, queues, extensions, call routing, IVR, and advanced voice features.
Unified Communications as a Service (UCaaS) extends this concept by providing a fully integrated collaboration platform that brings together telephony, chat, video calls, meetings, presence, file sharing, and team collaboration within a single, native and centralized solution.
In short, UCaaS does not replace the PBX, but rather includes and extends it, adding collaborative capabilities designed to improve internal productivity and team communication.