Not sure about data on time saved, but I average 2-5 minutes on each trip to the Go store in Seattle.
As for removing retail jobs, Amazon does hire a ton of people for every store to restock, answer questions, prepare deli foods, etc. So the jobs are likely moving to different parts of the store.
Also, I’m not sure what the official goals of Go are, but I always thought it was to remove friction during checkout. Not to save costs on cashier jobs.