Here is a little thought experiment: Imagine you're a mining pool operator and you and your operator friends control 60% of the hashrate. This means you can expect to get 60% of the blocks rewards, and also around 60% of the fees spent on transactions.

So, what if you started spamming transactions? Well, spamming transactions would cost you only about 40% of the fees you pay, since you can recover the other 60% by mining your own spam. At the same time you're building a huge backlog of transactions thus forcing the rest of the network to raise their fees. This is great news for your mining operation, as you will grab 60% of these increased fees. As long as you have a large enough mining share and enough transactions, the money you lose from other pools mining your spam might be less than what you make due to increased transaction fees. In other words, you can reach a point where this attacks pays for itself!.

Once you reach that point it just a matter of maximizing profits: You can reinvest your profits into the backlog you've built, using replace-by-fee with your original transaction spam to slowly increase the fees in the backlog more and more. Finally we will find out how much people are willing to pay for a Bitcoin transaction.

So, what's stopping the major pools from doing this? Maybe they just don't enjoy the thrill of conspiracy. Maybe they do and the current attack is a test run to see how that affects fees. Maybe they'll wait until the block reward halves and fees become a bigger part of the profit. I don't know, but I love the fact that this is even a possibility.