I think to solve this problem properly, we have to stop treating "build" and "test" as separate buckets of work, to be designed and scaled separately. It's all CI. As soon as you try seriously scaling out tests, you will run into build bottlenecks. To truly scale CI you need a scheduler that understands your build, test environment, and all the glue in between, well enough to schedule it in a way that actually speeds things up. That is very difficult and not something that even the best build tools can do - yes, even Bazel. Bazel can run tests but it's not nearly as good at it than at building.
Yes. Bazel is half of the equation. The other half which other enterprises rely on is the server side of Bazel's Remote Build protocol. There, the scheduler can be implemented with logic that accounts for sandbox sizing as well as other requirements/constraints.
