With the Nintendo Switch being both a, handheld and home console (hybrid); it will need to have an increase in hardware sales when factoring in the last 5-6 years of the 3DS and Wii U sales combined. A Switch Mini will probably take over the handheld console slot, but what's different is the same cartridge will be utilized on both devices (no need to purchase software twice). Less software being sold means more hardware sold is needed to offset potential losses from the previous generation. 80 million is being conservative, when factoring in 2 console's hardware sales.
This line of reasoning never made sense, even in 2017.
The whole Switch was setup to maximize profitability (unique development environment instead of being split, push toward higher margin digital sales and additional revenue through cheap DLCs, online subscription bring in recurring revenue etc.).
Switch is going to sell less hardware and software than Wii+DS yet the Switch era is going to be much much more profitable than the Wii/DS era.