No you're allowed to use Nintendo as a comparison, and I'm not saying your comparison is wrong (though maybe it sounded that way, I don't know). It's not an invalid comparison. It just seems incredibly redundant to use Nintendo games as an example, but then ignore that Switch is a much better comparison than the Playstation 4.
As for the games ... ?
Resident Evil 7 got an 86 on Metacritic and Resident Evil Remake 2 got a 93 (91 if you want to go with the PS4 score), and Resident Evil 7 sold 5.1 million in a year and nearly 3 months whereas Resident Evil 2 has already sold 4.5 million in just 7. (Sales + Reception 2019 > 2017)
Nioh and Devil May Cry 5 got the same metacritic score, and Nioh sold 2.5 million in 2 years whereas Devil May Cry 5 sold 2.9 million already. (Sales 2019 > 2017, Reception Equal)
Nier Automata got an 88 whereas Kingdom Hearts 3 got an 83, but Kingdom Hearts 3 shipped 5 million in one week and also was the best selling game of the year in the U.S. and Japan for quite a while, I think only losing that title in the U.S. after Mortal Kombat 11. (2019 Sales > 2017, Reception 2017 > 2019)
Mortal Kombat 11 shipped 5 million in one week and is tracking to be the best selling Mortal Kombat ever. Mortal Kombat 10 holds that now with more than 11 million sales. MK11 doubled the comparable sales of it's second month compared to every Mortal Kombat game in history. It's hard to say what Injustice 2 sold, but it sold 500k on digital storefronts for all of May, and it released May 11th, so I think it's fair to say MK11 is tracking better. (2019 Sales > 2017, Reception 2017 > 2019).
Sekiro sold 3.8 million copies with a little over 3 months on the market, and got a Metacritic of 91, making it better received and a bigger seller than Nioh (2019 Sales + Reception > 2017).
I could keep going but I think you're realizing how silly this is. It's all about how you match it up. I tried to be fair by matching up games that were in the same genre or by the same developer or publisher.
Don't get me wrong, I think to the general consumer market this year is a little worse than 2018 and 2017. You don't see as much heavy hitters as those years, especially early in the year - there's no Horizon or God of War comparison to be made for example, there just isn't really a replacement. But even then you have games like Days Gone which sold so well despite having very mediocre scores. Or the fact that games in nicher genres are generally getting more sales than other comparable games. I think this year is worse for system sellers , but it's still pretty comparable overall, especially when you consider this is the last year before the Playstation 5 comes out - the game releases are naturally slowing down, and yet 2019 still feels pretty close to those other two years. If anything, it might even be a better year in terms of quality, just not in overall popularity and quantity.
So again, I think a much bigger contributor to a sales decline is just saturation. Most people who want these games probably have a Playstation 4. Is some of it the selling power of the software? Sure, maybe. In fact, even on the software front we're at a point of saturation, as games like Watch Dogs or The Division are having sales that are generally worse, most likely because they are sequels to games that were received as lukewarm. But even then, if this year's release schedule was the release schedule for PS5's 3rd or 4th year, it would probably sell just as well as Playstation 4 did.
Also, Death Stranding being the only game of the year material to a large amount of people really has more to do with the general close-mindedness of what should be acceptable for a Game of the Year nomination. It has to usually be exclusive, usually be a huge seller, and usually be acclaimed. I mean, it's not even released yet and people are already betting on it. Devil May Cry 5 definitely deserves a shot at the nomination, but it probably won't get it, and Nintendo has released many games that will probably sell 5+ million lifetime, but won't get the nomination (Fire Emblem, Mario Maker, Luigi's Mansion, Pokemon, etc.).
Me saying PS4 is like Nintendo games about selling in no way means that Switch can't be the same or even better in doing the same. One thing doesn't invalidate the other. My comment was about a discussion i had yesterday about how much Nintendo games tend to keep their initial price because they keep selling and selling through the years (important aspect here..., years), and it came to my mind that with PS4 is a very similar thing too, but just with consoles. I don't know why should i need to name Switch in here at all. It's like to say that Zelda sells like Mario Kart and you come to me and say to me, "but it's Mario 3D game the one who is more close to sales to Mario Kart than Zelda!", well, yes..., or maybe not, but the original point still stands.
In regards of the second point. Yes, KH 3, The Division 2, MK 11 have sold a lot too, yes, but you even admit yourself, this year we have not seen really a system seller like MH World, Far Cry 5, God of War, Horizon Zero Dawn, Fortnite, ....It's a general consensus by almost everyone that 2019 has been worse than 2017 and 2018 in the same period. I don't think i'm lying in this regard. The rest of the year is going to be better, yes, but still nothing compared to the caliber of 2018 juggernauts like Spider-Man or Red Dead Redemption II.
It's just that i expected better quality games (critically i mean) in 2019 at the start of the year for PS4 to reach the numbers i expected by the end of the year, but it seems it wasn't as necessary. And yes, i agree with you, the main reason PS4 is selling less this year is saturation. That's what it makes this year sales so impressive. No home console (or if you prefer, console as a whole) has done 15M+ in a year after 5-6 years since launch still at 300$ or more. PS1, PS2 and Wii (and here i making a direct comparison indeed) were not able to do that, and DS was never as expensive.