You have begun with the incorrect assumption that your bitcoin miner will hash at a constant percentage of the network hashing rate for 730 days. I fail to see the usefulness of your graphs.
It makes no such assumption. As long as the network hashrate is at or below the lines on the chart, at any point in time you could buy a miner and end up breaking even after 2 years, of course constrained by the listed assumptions and the curves themselves, nothing else. If your point is that after 2 years the network may exceed those lines, thats actually part of the point, but it requires either changing constraints or someone betting > 2 year.
Let's dissect the data where all points converge @1239PH/s network speed and no cost for electricity.
1 TH/s is costing $700 now on your chart and bitcoin is worth $330;
1239 PH/s = 1239000 TH/s
Take the reciprocal of this 1/1239000 to find out your share of the block rewards for 1 TH/s and multiply that by the total number of bitcoins produced in a day.
total bitcoins per day for the entire network (ideal):
25 per block (reward) x 6 (blocks per hour) x 24 hours per day = 3600
3600 multiplied by 1/1239000 = 0.0029055690072639225181598062954 Bitcoins per day (ideal)
Now if you make that every single day for the magical number of 730 days you get 2.121065375302663438256658595642 bitcoins
and when we multiply that by the assumed $330 per bitcoin... drumroll please... we get $699.95157384987893462469733656186 or approximately $700 dollars for our 1 TH/s miner that we paid $700 dollars for 730 days ago! Which only works for a sustained network hashing rate of 1239 PH/s for a period of 730 days.
Your graph and your assumptions are flawed.
^that is my point
The assumptions are just that, feel free to alter them yourself. The graph isnt flawed, you just dont understand what its telling you, even after manually verifying the math.
What you fail to understand is that the network isnt going to keep growing magically if there is no ROI to be had. That graph tells you
when there is no more ROI to be had (or at least not within <2 year, which seems a reasonable horizon to me but feel free to change that too). Once you reach that point, who is going to buy or deploy more miners? Few if anyone, and if someone does, others will have to unplug, hence the network hashrate will remain roughly where the chart tells you it will plateau (for your chosen assumptions).
First of all, it's not my math, it's your math. You said you did not make the assumption that the network will remain at a constant rate for two years. Your math shows you did but whether you did so consciously or not is an altogether different argument.
Secondly, your charts have nothing to do with network growth as you have only made very narrow and fixed assumptions. So my failure to understand network growth based on your primitive charts is not at issue here.