Even with your description of a gradee A webapp that uses local state management effectively + shows syncing + queues with stacking + connectivity detection and exponential retries etc. I still feel like toasts can be useful to indicate to the user when are not in the "normal" state. I feel like especially mobile apps fail horribly at this, it is very normal to walk around a city and end up in dead spots. Having a clear indication from the device that we are longer in Kansas can be very useful. IMO toasts that plop up for successfull actions are often quite useless and redundant.