I guess you missed or don't understand this part: The bottom line is I can mine more coins for the same amount spent on cards with the RX 4XX/5XX than any Nvidia 10 series cards. Even after the cards are paid off, I will still make more coins going with RX 4XX/5XX than I can buying 1070's or any other Nvidia card and if the profitability drops, I will still be able to make a profit where a single 1070 won't.
and you don't understand that what is the advantage from lower investments AFTER all investments are payed off?
you still pay for your cards something (beside power)? after the cards gone after they are payed off and you have to buy again new one for lower investment?
How much money do you make with 6 x RX4xx/5xx AFTER all your investments are payed?
How much money do you make with 6 x 1070 AFTER all your investments are payed?
Just two simple questions
Your calculation would just be right if you permanently invest into new cards over and over again and throw them away after they are payed off
Two RX 4XX/5XX will make more profit than any single 1070 or 1080, whether the cards are paid off or not makes no difference in the profitability of the cards. A quicker ROI is an added benefit that comes from the cards being more profitable to mine with. Take ZEC for example, two RX 4XX/5XX will currently make $133 a month compared to $104 for a single 1070 after the power consumption is factored in.
https://www.coinwarz.com/calculators/zcash-mining-calculator/?h=620.00&p=250&pc=0.10&pf=3.00&d=3745339.65371322&r=10.00344334&er=0.11836001&btcer=2518.90000000&hc=0.00
https://www.coinwarz.com/calculators/zcash-mining-calculator/?h=470&p=150&pc=0.10&pf=3.00&d=3745339.65371322&r=10.00344334&er=0.11836001&btcer=2518.90000000&hc=0.00
If the difficulty doubles from where it is now. A single 1070 will make $53 a month vs $66 a month for two RX 4XX/5XX cards.
Which means it is more efficient and profitable to mine with RX 4XX/5XX cards than it is to mine with Nvidia 10 series cards.
https://www.coinwarz.com/calculators/zcash-mining-calculator/?h=470.00&p=150.00&pc=0.10&pf=3.00&d=6745339.65371322&r=10.00344334&er=0.11836001&btcer=2518.90000000&hc=0.00
https://www.coinwarz.com/calculators/zcash-mining-calculator/?h=620&p=250&pc=0.10&pf=3.00&d=6745339.65371322&r=10.00344334&er=0.11836001&btcer=2518.90000000&hc=0.00