I did some quick calculations for a friend earlier today and at current coin prices, for every dollar you spend on mining hardware (GPUs, mobos, risers, PSUs), you’ll make a dollar over the course of a year. That means it will take a year just to earn your initial investment back, excluding electricity. However, every year after that is profit on your investment, excluding maintenance and replacement parts. So, if you bought $50,000 of equipment today, after the first year, you be making $50,000 per year minus upkeep cost. But coin prices won’t stay constant. If price goes up, you make more money and pay off equipment faster, but prices could also go lower from here...
You failed to also account for the rise in mining difficulty. Even if the coins price stayed the same or even edged up a bit over the one year you projected, the difficulty would continue to rise so you would mine fewer coins each month, thus reducing your income.
With the rapid introduction of ASICs to most every algorithm and considering how fast ASICs are replaced and older ones become obsolete, also thinking you will continue making $50k each subsequent year is another false assumption. In today's mining world $50k of ASICs bought today would most likely need to be replaced by $50k of newer ASICs next year and so on, thus your profits will be very slim in between upgrade cycles.
If your calculations were solely based on using GPUs, with the Z9's out now and the E3's launching next month, GPU profitability will be next to nothing very soon. I predict by September the used GPU market will be swamped. So under either scenarios, GPU based, or ASIC based mining, the days of thinking you will ROI in one year and enjoy subsequent years of profit with paid off hardware are over.