It really isn't that abnormal.
NDS shipped an even more impressive 27.11m in a single fiscal year and then the rest of the lifetime was 25m.
Still, I guess we are part of opposite extremes when it comes to Switch LTD sales.
It is, in fact, abnormal. The DS had the steepest drop off of any system ever. On top of that, even in that scenario it's remainder was 92.5% of that 27.11 year, while you are predicting an even lower 89.5% for the remainder of Switch's life. The only notable DS software for that time period was Pokemon Black/White, while Nintendo is still planning plenty of software for the Switch. And the DS had gotten a price cut, while Switch still hasn't gotten one and has a whole lot of room to drop the price.
You are predicting the biggest cliff in history while all factors point to their being no cliff at all.