You're right re: the 75k/week figure. I mean, it's not hard to divide 3.12 million by 40 weeks. However, only March, October, November, and December managed to break that 75k figure, iirc

The problem with your reasoning here is that Nintendo appears to prefer using hardware restocks to encourage better sales for games at launch. Which means that - technically - artificial scarcity is at play here.

Hardware Sales
PS4
DQXI 
W1　93,954
W2　52,819
W3　38,015
W4　39,970
W5　26,335

MHW
W1　140,432
W2　109,286
W3　 70,172

That doesnt seem true at all giving the fact some of the older games are charting even better then ever way after release. 

Ex: zelda (game selling 5-7k a week before holidays and now selling 10k+ week)

Pokken

ARMS

12S

Fifa

DBXV2

MK8F

Splat2

All games that aelling better then it was initially doing, even with a bigger library compared to when each game came out

Maybe the Switch has already peaked in Japan, who knows.

I mean, even the Wii did peak there on its first full year, and I suspect the same happened to the GC and the N64 (for the N64, I'm sure it did happen in the US at least).

Your examples are proving my point. Name me any notable games aside from Mario and Rabbid that was release in Q1 2018 currently?
Evergreen titles will continue to sell that's given however he/she specifically asked for Dragon Ball and MHXX. 
If I am a Switch owner I will first look at Splatoon or Mario games or Zelda.  That's the system seller of Switch.
Xenoverse and MHXX( this one is a bit different) are companion game so the more option this user has it will be difficult for this type of game to have legs. You will not see people buy a Switch for Xenoverse for example. If DBFZ, Naruto and other fighting games from Bamco do you think it will not cut the legs of Xenoverse?

Unless you can provide me a better explanation on why the games you listed are selling? Aside from evergreen titles.

I mean no game last forever so theyll fall off the chart unless your one of the leggiest games. I was more referring to the fact that some of the games are selling previous higher on a weekly basis (like pokken/dbxv2) from what they were originally selling at launch. Zelda of course is the most notable.

I guess i was saying that more releases came out after (pick which ever game ex FEW) released and its still selling the same if not better than it was before.

More games come out your right, itll fall off. I guess i was trying to make another point and failed to explain it right....

Yes you are right but you will need to take in to account that DBXV2 and pokken was sold out as well weeks or a month ago. 
Dragon Ball, MonHun and Pokemon are well known brand in Japan but at the same time with limited selection people opt for game that's available on the market. 
I am not saying it as a negative though. As for my observation that's the most logical explanation. Switch is a hot item. People want to play more games on it.

Another great Switch week.

It's funny that people thought the week 1 ps4 boost would be only 70-80k. Blew expectations out of the water.

I expected is will do over 100k week 1, look like i was wrong