There's a fundamental lower bound on the energy usage of desalination. The current state of the art - reverse osmosis - isn't perfect, but we should not expect factor-of-ten reductions in energy usage.
Somewhere like Austin could definitely use solar energy to boil water couldn’t they? Not only solar panels but those mirror/reflective panels into a concentrated spot to shine?
Austin is ~150 miles from the sea, so you'd incur substantial pumping costs.
Solar thermal "flash" desalination is one of those things that sounds obvious but is hardly ever done, so I suspect there are good reasons for that.
We have heaps of fresh water in NA, I guess except in these areas (many of which seem to be in places where fresh water is scarcer). I think for many of these, it is still cheaper to bring water from a fresh water source than it is to bring it from the ocean, even if there is no desalination cost.
Pretty sure lack of fresh water supply isn’t the problem in Philadelphia.
There was (almost) never a problem running out of river water. Freshwater (i.e. river) treatment is peanuts compared to desalination. It's pumps and pipes that cost most, not filters.