It probably doesn't make sense to do infrastructure -wide duplication for a greywater system. That would be a lot of pipe and possible leaks in places where that resource isn't needed.

Smaller loops make more sense for specific needs like this. It just needs to be legislated - over a certain size, you need to pump, filter if required for your application, and then dump in accordance with whatever rules we set. If local governments want, they can subsidize this through tax breaks - we already have robust systems for giving corporations money back, we just need to tie it to the types of performance we need to see, whether that be environmental improvements, job creation/retention, etc.