Just because you are using AWS/GCP/Azure doesn't mean that you don't need sysadmins. They are called DevOps engineers then.
In my experience, whenever someone actually had to "foot the bill" for the AWS premium, your argument that the cloud is cheaper overall came under a very heavy scrutiny, including justifying those extra "9s" that increase administration effort.
So administration costs aren't zero today, but I suspect they'd be higher if they moved off the cloud.