There are plenty of places around the world with western style toilets but flushed with rainwater or recycled washing machine/shower water instead. That seems like a fairly sustainable solution that already works?
Yes, those are better solutions, however still not really sustainable, and without incentives to build the accompanied infrastructure, it's never going to be sustainable; whereas solutions like the composting toilets are fairly easy to implement - FWIW ~ that's what we had before water flush toilets, isn't it?