"Anyone thinking that the Switch and the PS4 are not stealing sales from one another is just being delusional. There are people out there in the world, or in this case Japan, that are choosing between the two. If both didn't exist, they would get the one that did. But since they both exist, some people are choosing one or the other". Point is simple, PS4 sales in some way would probably be better in Japan also like in rest world where PS4 is notcible YOY up, if Switch isnt so much popular in Japan.
How exactly thats make my argument invalid, fact that 1st party games are best selling Switch games doesn't mean that they are selling only to Nintendo fans. Offocurse that great and incredibly popular games like Zelda BotW, MK8D, Splatoon 2 and Mario Odyssey can be bouth and buy people that are not Nintendo fans. Like I wrote, you can buy Switch for Zelda BotW, MK8D, Splatoon 2 and Mario Odyssey even if you are not Nintendo fan because of great reception (I mean Zelda BotW and Mario Odyssey are one best critically acclaimed games ever, MK8D is one of best MK game ever and Splatoon is strongest new IP in Japan) and popularity of those games, huge Switch popularity and hype in Japan, or portability of Switch, and Switch is also home console in same time like PS4, can easily affect someone to choose Switch instead of PS4. We already seeing plenty of people that saying that Switch is their first Nintendo console in long time (in couple of generations) or even first Nintendo console ever.
The problem in your argument is you cannot accept that the 2 can co-exist.
At the moment they don't share the same library. Even with the future releases it's still different. MHW is just a month away do you think Switch will steal sales from those people who are anticipating it?
And there's a reason I see 2018 as the real test because that is the time where 3RD party devs from Japan will start to release games for the Switch be it exclusive or mulit plat from the PS4.
You are also ignoring sales trend just take a look at the sales trend in 2015, and 2016. You are also ignoring the climate of home console in Japan. 2014 is horrible.
Week 40 2014 - 7,350
Week 40 2015- 46,177 (price cut, Night Azure, Winning eleven)
Week 40 2016- 31.071 (blazblue, Slim 3rd week)
Week 40 2017- 25,398 (Dragon's Dogma)
Week 41 2014- 11,697 ( Fifa,Driveclub)
Week 41 2015- 30,349 ( UC collection, FIFA, post price cut)
Week 41 2016- 30,154 (Tombraider, Slim 4th week)
Week 41 2017- 22,706 (Shadow war)
Week 42- 8,046 (nothing)
Week 42-25,699 (nothing)
Week 42- 27,069 ( Battlefield)
Week 42- 31,278 (GTS, City shrouded, Evil within, Itadaki street)
Week 43 2014- 8,942 (evil within)
Week 43 2015- 22,801 (nothing)
Week 43 2016-30,375 (Berserk, Titanfall, World of FF, SAO)
Week 43 2017- 24,653 ( taiko, asscreed, ark)
Week 44 2014- 8,906 (oneechanbara Z)
Week 44 2015 - 22,104 ( God eater)
Week 44 2016- 32,041 (COD, Dragonball Xenoverse, Atelier)
Week 44 2017- 28,823 (COD, .Hack)
Week 45 2014- 10,078 (nothing)
Week 45 2015- 33.442 ( COD BO II)
Week 45 2016- 93,512 (Pro launch, fate exltella)
Week 45 2017- 26,058 (NFS)
Week 46 2014 - 15,537 ( COD advance warfare)
Week 46 2015- 27,311 (Ass Creed syndicate)
Week 46 2016- 38,026 (Post pro launch and Fate Extella)
Week 46 2017 - 26,302 (star wars battle front)
Week 47 2014 - 12,430 ( as creed unity)
Week 47 2015- 29,052 (Star wards battle front, Atelier Sophie)
Week 47 2016- 42,102 ( Pro 3rd week, and SD gundam)
Week 47 2017- 26,909 ( only Hatsune miku a physical release from a free demo.)
Week 48 2014- 13,489 ( Dragon age Inquisiton)
Week 48 2015- 26,598 (Blade Arcus)
Week 48 2016 - 118,350 (FFXV with LE edition, Watch Dogs II)
Week 48 2017- ???
Week 49 2014 - 15,971 ( Nothing)
Week 49 2015 - 35,782 (Bloodborne Complete)
Week 49 2016 - 92,717 (Yakuza 6 with LE, Last Guardian, Post FFXV)
week 49 2017 -???
Week 50 2014 - 68,041 (GTA V)
Week 50 2015- 38,497 ( Steins gate, Gravity rush remastered, Holiday)
Week 50 2016- 79,348 ( FF 3rd week, Post Yakuza and Last Guardian Holiday)
Week 50 2017- ???
Week 51 2014- 30,951 (post GTA V, Holiday)
week 51 2015- 53,123 ( Fall out, Holiday)
Week 51 2016- 100,238 (Holiday, Slim first holiday)
Week 51 2017 -???
Week 52 2014 - 33,150 ( Holiday)
Week 52 2015 - 53,023 ( Holiday)
Week 52 2016 - 83,337 (Holiday)
Week 52 2017 -???
Week 53 2015 65,427
See the trend? PS4 is ranging from 20K-32K from 2015-2017. PS4 is selling like PS4 if you follow the trend. 2015 and 2016 Switch is non existent but the sales are still the same. There are spikes if there are notable games that being released.
Just looking at Q4 2016 it's very massive in terms of releases, price cut, games. PS4 2017 Q4 is weak compared to q4 2016.
The only week it's up was when GTS was released. It's possible to be up again on Week 49 with MHW LE PS4 pro, MHW beta, Yakuza Kiwami 2, Kamen rider, EDF5 and dead Rising 4 but after that it's unlikely for PS4 to be up again.
So Is this really the case of Switch stealing sales from the PS4 or is it just PS4 doesn't have a strong Q4 and sales are reflecting it? because historically speaking and the sales trend from the previous years even without the Switch it will still sell the same.
For the 2nd part. I misread sorry but you cannot really blame me.
"Point that Switch are buying only Nintendo fans in Japan is wrong."
It's really hard for me to understand that one.