>how was anyone expected to make changes to the build definition if this was the only way? Wait a day to find out if it worked, and break the build for everyone if not?

You could build a subcomponent of Windows source locally, and 99% of the time, you knew which subparts of the source tree to build locally to have confidence that you wouldn't break the build.

The problem was that if I was doing something exotic nobody has tried before, I'm suddenly in the 1% case where I no longer have confidence that building locally will eliminate all issues.