you are 100% in asking the question and difficulty change play a big part in this. The thing I am taking into consideration - for the difficulty to increase by 10% or 20% per month the network has to add that % hashing power. Now with an estimated 16-17Eh in the network... calculate the number of S9s needed to do that. It will plato imo but continue going up. How much ? Who knows.
edit: also keep in mind that other SHA-256 coins take some of the hashing produced.
That's a fair point. However, I wonder if the hashing power won't grow at that rate or even higher. With all of the ASICs in the market and more powerful and plentiful ones coming, it isn't hard to imagine the network growing exponentially.