If people aren't willing to pay what they're charging then they'll go out of business, grey market or not.
And again, we have the people who were (historically) willing to pay £35, but no longer have to, since the game is on a grey key seller for £20 from Day 1.
This is where the revenue loss is realistically occurring. Perhaps not from those who categorically won't spend more than £15 on a game.
I'm willing to bet there are a lot of people (esp kids) who have the money - and inclination - to buy lots of games at £35, but no longer have to, since the grey market give 50% discounts from release day onwards.