Probably because 3DS is going to pass 70 million and was worse in nearly every way.
Also, by what measure is Switch selling 100k a week? It sold 100k in its absolute lowest week, and 128k in its second-lowest week. Even in its worst month, May, it averaged 140k. Judging from the increased baseline in Japan's sales, it's probably selling around 160k per week now.
And, sure, I guess it's possible that only 15 million people want a Switch, and sales will fall off a cliff after selling 11 million in its first year. But that is something that has never happened before in video game history, so why would you expect that for Switch? You might as well expect PS4 sales to drop to 3 million next year after selling 20 million this year.
We have more than enough data to know that Switch is off to a great start.
3DS had multiple hardware revisions though. A lot of people wound up buying two 3DS units. I can see the Switch getting a remodel, but not five different units like the 3DS had. 3DS had 3DS Original, 3DS XL, New 3DS XL, 2DS, and New 2DS XL.
Well, Sony hasn't had a home console bomb just yet. That's the difference.
I agree that Switch is off to a great start. I've been estatic about it for the past four months. Go Switch go!