You think it would be tough to sell a pure water window cleaning in a place where anyone can do it themselves with a normal brush and their own tap.

It better be a very clean brush

Yes did a home with 124 tds with the new water bar jet steam
www.youtube.com/watch

This is one of those maybe you can and maybe you can't. If you are getting your water from the same place every time and know that it doesn't spot at a low TDS then sure. Of course you are missing out on the benefits of DI which imparts a slight electrical charge (interestingly enough the charge is the exact opposite of the dirt on the window almost like they planned it or something) that helps draw dirt off the window.
In the works of the real Mythbusters - Plausible
Of course it's not really practical for those who clean using a system drawing from many taps or do several houses in one day.
So in the real world this one is actually - Busted