> we buy one or two very expensive machines that run CI and builds

This unfortunately does not work for orgs that have, say, more than 20 engineers. The core issue is that once you have a test suite large enough to have ~30 shards, you only need one engineer `git push`ing once to saturate those 1-2 expensive machines you've got sitting in the office.

The CI workload is quite amenable to "serverless" when you get to a large enough org size, where most of the time you actually want to pay nothing (i.e. outside your business hours) but when your engineers are pushing code, you want 1500 vCPUs on-demand to run 4 or 5 test suites concurrently.