Super Cruise, GM's Autopilot competitor, is planning to be fleet-wide by 2020[1]. But that is L2/L3 stuff, which is different than the L4 technology Cruise is focused on. You can't put L4 into GM's cars because it's not ready - it's probably dangerous and I'm doubtful it would be cost effective given the prices of sensors and the fact that you need to turn the car into a rolling datacenter.

[1] https://mashable.com/article/super-cruise-cadillac-ct5-sedan...