As the linked article says, modularizing and living in separate repos was the plan of record for a while. But after evaluating the tradeoffs, we decided that Windows needs to optimize for big, rapid refactors at this stage in its development. "Easy to make sweeping changes" also means "easy to clean up architecture and refactor for cleaner boundaries".

The Windows build system is where component boundaries get enforced. Having version control introduce additional arbitrary boundaries makes the problem of good modularity harder to solve.