Conquered the US? Quite an odd choice of words when comparing monthly sales of a 2 year old console to a 5 year console that is still $299. Let me know when the Switch has passed the PS4's LTD, then we can talk about conquered.
Good, see you in about 3-4 years then.
Switch sold over 1.4M in the US YTD but the PS4 less than 1M. While the PS4 sales have dropped, even in it's best year it couldn't have kept up with the Switch sales from this year. And the Switch is still at full price, so there's still much space for pricecuts to boost sales later on, ensuring that the Switch will have a long shelf life, or at least longer than many believe.
In other words, I'm pretty sure that the Switch will outsell the PS4's best years in the US during the next years, making the advantage melt down fast. How long exactly it will take depends on how well and long the PS4 will sell from here on out, but if the PS5 really has backwards compatibilty out of the box, there's not much reason anymore to buy the console (at least not a new one) after the PS5 is out. So I expect the PS4 sales will drop fast after the PS5 is out, much faster than the PS2 and PS3 did.
The Nintendo eShop rating Thread: http://gamrconnect.vgchartz.com/thread.php?id=237454