in 2 years it will probably have eclipsed Wii already
PS4 shipments were at 60 million as of March 31, 2017.
Sony projects 18 million (down 10% from last year) for the fiscal year ending March 31, 2018 bringing the total to 78 million.
The fiscal year ending March 31, 2019 will likely be around 15-16 million bringing the total to 93-94 million.
From there PS4 will still need to ship another 8-9 million so it will likely surpass Wii right around the end of 2019.
When the herd loses its way, the shepard must kill the bull that leads them astray.