Why would 5G be a big deal for self-driving cars? Any self-driving that requires ubiquitous low-latency communication is hopeless. Self-driving is a vision problem, and it has to work in a messy world.
Do human drivers have instant car-to-car communication? No. So why should a self-driving car have it?
Self-driving cars will never happen if the fundamental assumption is that every single vehicle on the road is able to actively communicate like that.