20 mil. unit sales in a FY seems to be the metric everybody is looking for and everything slightly below it is sniffed at. I don't know if the Nintendo Switch will ever sell 20 mil. in a FY but I do think that it will sell more consistently over a longer period than previous Nintendo home consoles. There are a few hints for that:
1. Nintendo themselves have said that they aim for a longer lifespan than usual
2. Back in the NX days, Nintendo said that they look at the new console as a family of devices like different generations of i-phones and i-pads which in turn could mean several generations and variations of Nintendo Switch SKU's, prolonging the life.
3. Still no price cut in sight, a good indication that the console is healthy and vibrant and has a long life
Of course, it's always the aim of a company to have a product successfully on the market for as long as possible. However, so far, for home consoles, Nintendo had a tendency to slow down in their 4th year, partly also self-made because they focused on the development of games for the new console in development. I think Nintendo wants to avoid that this time. Another advantage is that this time around they can focus on only one console, no handheld to support anymore. Of course, there are various unknown factors which can work against your aims but tendentially, I believe the strategy of consistent, steady sale over a long period could work out.