Depends how much water costs; in the UK water is cheap compared to the cost of a plumber, but the UK is famously moist. I wouldn’t be surprised if plenty of more arid environments would find the cost going the other way, and preferring some combination of rainwater collection, greywater reuse, or non-water-based toilets.

You are absolutely right, however, as ben_w below also pointed out, while the economical / environmental impact might not be justifiable in our western bubble, it might be outside of it.

There are still places on this planet where clean drinking water is a scarcity, and therefore - along multiple other reasons - water-flush toilets and accompanying infrastructure cannot be (easily / justifiably) built.

However, as we can see there are different solutions to the problem itself (health-risk free, humanic way of taking a dump) without using up gallons of water.

My house has a non-potable water supply used for irrigation, and is in fact required by the town to. I know the same is true in some parts of Utah and probably other places. In such cases, also requiring toilets to be supplied by the non-pot line would be a trivial cost that would pay for itself eventually.

If you’ve already got it then it makes sense to use it, but this doesn’t remove the cost of building and maintaining separate infrastructure at all.

In some cases it might be worth it or necessary, in others I think the process of processing & delivering potable water could be made (or is already) so efficient that building & maintaining separate infrastructure for non-potable water would be a net negative both in terms of cost and environmental impact.