Suppose a company invented an aids vaccine, and then charged gay folks and injection drug users 20x, because of hey - those at higher risk are willing to pay more for protection would that be a great thing?

Or let's go back to the Paris example. If everyone is paying the most they can afford, that leaves less money on the table for restaurants, cheese and wine when actually in Paris. It's 'great' if you only look at the market dynamics of flights and ignore the economic/social benefits of cheap travel.