If only that were economically true. The people who buy Fastrack are going to buy Fastrack regardless, they'll be overwhelmingly price blind to the increases. There may be some grumbling and moaning, but the vast majority, if not all, will still fork up.
The people who buy Fastrack passes are doing so because they want that experience. They believe it gives them an extra privilege, right, or access in exchange for cash. These are people who value their perceived sense of time, and exclusivity, more than their money, an increase of £20 per ticket at the high end, when you're already considering spending £110, is nothing in the grand scheme.