[swift]

There are definite tradeoffs here, but one thing to consider is that dev tooling generally can't benefit from that kind of massively distributed parallelism. If you make a change to a widely-included header file, code intelligence tools suddenly have to do a huge amount of work to reevaluate their internal model of all affected translation units. This can really result in an unpleasant and laggy editing experience, even on medium-sized projects. That's the kind of thing that modules can help with; I certainly wished fervently that they were already a thing when I used to work on C++ static analysis tools years ago.