In the future we will have generators that will _not generate_ until there is a certain amount of high fee blocks waiting, and then they will automaticity turn on their massive generation capacity and quickly generate the block. This leaves the average difficulty relatively low, and makes a strong incentive to include larger fees. These generators plainly wont include any low fee blocks, low fee blocks will have to wait around for a charity block.
How will the fees build up if the smaller generators are constantly processing them. there isn't going to be a sudden influx of high fee transactions... The large generators will simply be leaving all the profit to competitors. And if the competitors are snapping up any transaction no matter how small the fee, there is little incentive to pay a high fee in the first place.