That's quite the drop you expect, an average of 10-15k units per week under last year's sales over the course of the next ten weeks.
I am not even sure if the current lead of ~350k units will be lower in ten weeks time because Japan's preference for handheld consoles tends to be overstated. The original Switch may cost more, but it also doubles as home console, so it will remain appealing. Fire Emblem: Three Houses is coming up, so weekly sales should remain above last year's weekly sales or at least hover right around last year's weekly sales in the coming next ten weeks.
I didn't check last year's sales when I wrote that. After having a good look at the numbers, I agree that that's too much of a drop, and I didn't take FE:TH into account. So now I rather think it will drop to about 300k, and that pretty much all of it comes from after Obon week (I also expect Nintendo to open preorders only right after Obon week to not loose out sales that week).
350k is possible if FE pushes high enough to compensate for all the "lost" sales until the new model comes out, maybe even more than that. I guess we'll have to wait and see for the results.
The Nintendo eShop rating Thread: http://gamrconnect.vgchartz.com/thread.php?id=237454