It's not really that surprising: the target audience is the same; and despite the Switch's huge success, PS4 and XB1 didn't go down.
It was "obvious" that those consumers were massively buying a Switch.
There are two things that i think should be taken into consideration:
Switch was lucky to come out at the time it did (it had 3 years of a userbase to take advantage off and, despite Switch's success, it remains to be seen if it can go from secondary console to primary console to that audience. If achieves that, it will be a bigger win that whatever it sells in it's lifetime.