I found most large companies who use cloud at scale have a team/s managing cloud, providing tooling around management and deployment or even full on PaaS solutions on top of providers. So you might not have you traditional sysadmins but you have a team of what recruiters like to call devops engineers instead.

The main advantage of cloud is the flexibility, treat resources as ephemeral, being able to click a button and get more/less resources. You don’t need to wait for a server to ship and be installed in a dc, if you don’t know what specs you need just twiddle the dials until you get something that performs as you want. It allows you to deploy / release quicker and easier.

It’s possible to architect so the cloud is cheaper but almost never happens. Optimising is intensive, often it’s called over engineering by pm’s/product owners. Costs normally start out low, blow up as products ramp out feature after feature and teams start onboarding, if performance is an issue it’s easier to increase an instance size instead of profile and change architecture. Only when the person paying the bill says this is to much does architecture and other optimisations happen, sometimes at a point when it’s hard to retrofit.