Antminer S9 ROI: Bitmain vs Retail over 1, 1.5 & 2 yrs

This is in addition to my previous chart on miner roi for different Antminer models. The S9 is the only model making a profit now and is the only sha256 miner currently used by Bitmain according to news articles. My previous chart only looked at ROI for retail miners. Here I show estimates for Bitmain (area plots) itself who pay less for their mining equipment than their customers. I also compare with retail miners (lines). The three lines/areas represent three different amortization periods for mining equipment: over 1 year (green), 1.5 years (blue) or 2 years (purple). The longer the period over which you spread your costs the less dependent you are on BTC price so long as your miner does not become obsolete before the write off period ends. previously I only considered a 1 year write off period.
Lastly after some research I determine the average lifespan of a miner is just 1 to 2 years (

Area plot: Bitmain
Line: retail

Elec: 0.06 USD/kWh
Model: Antminer S9
Hrate: 13.5 TH/s
Unit cost: USD 500 (Bitmain), USD 900 (retail)
Operating time: 24 hrs /day, 365 days/year