Yes the Xbox One isn't quite just quite as weak in the US but but does that matter that much since there were some cases where the PS4 nearly doubled/doubled on the Xbox One in some months ?
Why would Nintendo gamers be interested in a PS4 ? It's clear that almost none of them are interested in a lot of Sony's first party games nor are they interested in third party games so it's most likely that they would just quit gaming altogether.
With the xbox one it isn't clear what things will happen. If they do continue making games they'll be releasing them on PC from now on which would mean that Halo as well as other Microsoft titles will be released on there. If that happens a fair portion of xbox gamers will be migrating to PC or will be returning to PC gaming because of this. There's also small portion of Xbox gamers who are also disinterested with a lot of playstation exclusives so I can see them also going to PC rather than continuing their habits on playstation plus I also happen to share their sentiment about PS exclusives too.
Causation does not imply correlation. Not everyone will opt out to buy the PS4 if nothing else exists.
This is a sweeping judgement that is largely based on an Internet stereotype. Poor third-party sales on NIntendo consoles does not imply that Nintendo fans do not and would not want games not made by NIntendo if the Wii U did not exist. Their preference for NIntendo games does not imply a dislike for all others, even if some might seem like that. Many NIntendo fans have a secondary preference for another console (and buy ither games there) and/or like NIntendo-like games not made by Nintendo (i.e. Rayman, Kingdom Hearts...). I personally like JRPGs and some Sony exclusives, which I for the most part can't get on the Wii U.
Your conclusion that most NIntendo gamers would quit gaming without NIntendo is actually quite ridiculous. This might be the case for some, but you make it seem as if the demand for interactive entertainment would largely disappear into a void. What is to say that those who don't already wouldn't just develop an interest to NIntendo-like ganes on other platforms? Furthermore, you seem to imply the assumption that NIntendo's non-existence would not cause another company (lkely a Japanese one) to fill that void with similar games,
It certainly is not "clear"; that's just your opinion.
Aside, you seem to have this belief (that a lot of PC gamers have) that consoles mostly only exist for exclusives and that, without them, consoles would die quickly and everyone would migrate to PC/mobile or quit gameing entirely. That a brand is almost worthless without its exclusives (no matter how much it associates itself with third-party games). That people don't really value a budget properitary console built specifically for gaming, and that every would readily accept and embrace the PC environment if only we just got rid of consoles. Do you really think that the general consumer sits there deliberating intensively over which consoles have which exclusives, "objective" cost-to-value ratios, mulling over specs and genres, and so forth? Or do simply look and choose what is popular convenient, affordable, and looks fun? Gaming devices have different properties which influence how they appeal to different people. That goes beyond the simple "numbers games".