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