This is provably false. Customers are anything but rational, and pick things out that play against their best interests all the time.

Be it due to fashion, social pressure, brand recognition, cultural norms, et cetera and so forth.

I doubt it's provably true or false, as psychology tends to be.

Largely I agree with the OP though - treat your customers bad enough and unless there's something stopping them, they'll go elsewhere. That's how the free market works.