The rub is all in the "affordable." Chemical processes require energy. Energy costs money (even renewable energy like solar -- initial investment, dispatching, maintenance, there's real cost there). So, desal costs money. The key to cheap desal is cheap energy. Which, indeed, hasn't been cracked yet.
Oh, there's also the teeny tiny cost problem of transportation. Want to build a 450 mile pipeline to pump that desalinated water to Midland from Corpus? A decent-sized pipeline costs anywhere from $5 million to over $10 million per mile (and that $5 million number is way low from what I understand these days). Even if we split the difference and say it's $7.5 million per mile, that's around $3.4 billion....just to build it. Now you have to operate it (takes energy to pump water uphill) and maintain it -- a shitload in opex every year.
There are solutions to every problem we have (energy, water, etc.). All it takes is ungodly amounts of money. What we're searching for are cost-effective solutions, and those are really hard to come by. Not criticizing your post, BTW -- you acknowledge the problem. Just building on it.