All PLCs are configured to communicate to the Ignition Gateway, which usually resides on server-class hardware for large installations, however could run on a basic PC depending on your application requirements. Keep in mind this is actually best practice. Other platforms (WonderWare, FactoryTalk View...etc) will centralize PLC communications as well as PLCs are limited in communication bandwidth. Having numerous SCADA/HMI connections to a single PLC can cause performance issues.
Ignition clients are launched to run the actual application(s) you wish to use. The clients communicate with the Ignition Gateway. Never directly to a PLC.
So, you are correct. Should you have an issue with the Gateway for whatever reason all your clients are down. Ignition does offer options like local-client fallback, edge devices and redundancy to maximize your uptime.
However, if your network install is already a hot-mess no software solution is going to be able to withstand downtime because of it.
I suggest you review the manual, and inductiveuniversity.com start with the architecture section.
https://docs.inductiveautomation.com/display/DOC81/Standard+Architecture