I don't think it's that simple unless I misunderstand how GCP Pubsub works. I don't think GCP PubSub will give you deterministic delivery order within a partition the way Kafka will: https://cloud.google.com/pubsub/docs/ordering
Are you saying GCP pubsub have bad reliability? I never used it but would be surprised if it was down for several hours each weak or would just drop your data.
One of the teams was complaining about behavior on load. Now another team will migrate that flow to Kafka. Another team was complaining about reliability/speed. I think eventually they stopped relying on that.
