I'd argue due to the size of the install base and the appetite overall I've seen for it since it revealed mechanics that are reminiscent of BOTW/TOTK, achieving 5M copies sold during the first quarter + Holiday season is doable. I don't know if it'll have good legs due to the overcrowded market presence of 1st party games on Switch at this point, but it'll do good.
Switch Friend Code : 3905-6122-2909