I don't know what they could've done. If they made it a more traditonal console to compete with PS4 & XB1 then it would've done better than Wii U, but it wouldn't have reached Wii levels.
Wii was a viral product, and it's hard to make viral products one-after-another. I don't know why anyone believed they somehow got the magic touch and would keep putting out products as hot as DS/Wii.
If a company is steadily growing it's user base then you know it has a solid business strategy and can probably keep executing well. Nintendo is a company that was struggling to keep it's users, and suddenly had a surge in users on DS/Wii that they didn't even expect, and many of those users were not particularly loyal to Nintendo or to gaming, and as common sense caught up most of those users did not buy again.
