Comma almost certainly won't get widespread adoption until it has enough miles driven on it (at least a billion?) for people to have enough data to start talking about its safety record. So that means it'll be stuck in the realm of enthusiasts for quite a long time.
If it does prove itself, though, I agree with you that it could be a game-changer for people who have recent enough cars to be supported, but not have their own driver-assist systems. And that's probably a lot of cars, and a lot of people.
[0] Although, with the exploding batteries debacle...
I think safety has already been proven, and I don't think that's preventing widespread adoption at all. It's not like most people even know about this product. It's currently being purposely kept down-low in order to only attract a certain type of tech savvy person who is more likely contribute to the project in some way (pull requests, bug reports) or at a type of person who at least won't need heavy hand holding. A large mainstream consumer audience for a dev kit is too much for them to handle, and would just be a distraction.
Autopilot is iOS. Openpilot is Android.
Far more devices running Android than iOS in the world. And unlike consumer tech, the average car on the road is over 12 years old.
So there's a long window of opportunity where we might see more people choose to upgrade their current vehicles to self-driving, versus those choosing to buy new vehicles with inbuilt self-driving hardware.