This statement (as it is) is false. 

If you modified your statement with one word, it might be true. 

Don't sales of most consoles (and most other consumer e lectronics) resemble a normal curve? If that is the case then PS4 being in the middle of its life should be close to its peak sales and Switch should still be climbing. 

It is true. Where are the majority of 3rd party games? No Far Cry 5? Where are the games? Down graded ports of a handful of games don't really count for much.

Please, not if it replaces the traditional model. I don't want to pay more for a system with more compromises.

If Sony ever ventures into handheld gaming again, it should be when technology is mature enough that they can create a separate handheld or hybrid sku that shares a library with the base home console unit.

I'd love to have a "PS4 Go" to go with my PS4 Pro.

But I won't be holding my breath.

Taking a current gen game and dialing it back to last gen doesn't mean it's running current gen.

it can be something similar, Sony just need to do a PS4 lite with less specs in order to be a PS4 portable able to play PS4 games. And obviously it would be sold separately.

Just like there is a patch for games to run from the original PS4 in order to use PRO capabilities, they would also have to do a patch in order to run on the "PS4GO"

PS4 has had two price drops in the US. In late 2015, it received a $50 price cut (price reduced to $349) and with the introduction of PS4 Slim the price was further dropped to $299. In addition, the non-permanenet deals (such as the $200 Slim during November of 2017; $250 Star Wars bundle of December of 2017) all helped to substantially increase sales for the system in those respective periods.

As for Japan, it is true that the last official major price cut for the stand alone system was in 2015 which reduced the price to ¥34,000. However, since the Switch launch most retailers (at least online going by Bic, Amazon, and Yodobashi) have been selling the PS4 at the RRP for the Switch (¥32,378), with some retailers (namely Amazon) going lower. For example, the Amazon listing you posted has the price at ¥34,000, but upon clicking the listing, the actual price is listed as ¥ 30,895 which was already down from ¥32,378 (which is odd considering the system is out of stock).

The problem is that the terms "current gen" and "last gen" are fairly arbitrary. In addition, using the scaling down of certain games to have it run and the inability of the hardware to run certain games does not mean it is "last gen". By the token of your argument, a game like Doom is not current-gen on a system like the base PS4 because it can run at much higher resolutions and with better graphical effects on a mid-to-high end gaming PC. Beyond this, what about the many games that are not really graphically intensive but released only on recent consoles? Are they last-gen as well?

Also, how could you be sure that Doom and Wolfenstein II would run on a last-gen system. The Switch's raw power in handheld mode might be comparable to that of a last-gen stationary system, but its GPU architecture is fairly modern and matches the features on the current-gen stationary systems. Engines that rely on software APIs base around the newer hardware features might not scale to PS3 and Xbox 360 but might do so on Switch because the GPU has the necessary features for the engine to run.

Still the same game where it matters most.