What am I missing here?
You are missing that
difficulty can't rise forever with 15% per increase.I agree, it can't rise forever. The rises will taper off and we'll reach some rough equilibrium of hashrate.
It doesn't have to rise forever at 15% though. Only for the next 12 months,
or less if there's a couple of bigger leaps early on, or if your miner arrives at the end of Sept. instead of the beginning.
This time last year the network hashrate was 150TH/s. Today it's >600x that.
You don't think we can see a 30x rise over this coming year with all the manufacturers now on the scene?
Have you thought about the amount of money needed for last year's network hashrate jump and the money needed for next year's jump? It's a big big difference. Jumping from 150TH/s to 120PH/s is not the same as a jump from 120PH to 3600PH/s. If we assume a 0.5$/GH * 3480PH= 1.74B$ needed for this diff jump. Don't really think that there are so many money that are just waiting to be spent on bitcoin hardware.