PS4 is going to start with a 50M or more head start. It's sales pace is only going to pick up. Look at any console sales, the lower the price point, the higher the sales. PS4 best selling months are ahead of it. "Any kind of success" for the PS4K is not going to stop that. The PS4K will not outsell the PS4 for any sustained amount of time and will not have a higher instal base on PS4 until the PS4 is no longer supported.
PS4K is just another SKU to optimize for, and with so much similarity in hardware architecture, it's easier than ever. No one is going to have to outsource that like the did for PS4 vs. PS3, it will all be handled in house.
No one thinks that the existence of the XB1 has made the PS4 shit, or the NX coming out is going to make the PS4 shit, those are all additional platforms to optimize for. The hyperbole about developers not being able to optimize on SKUs, especially since they are already doing it is ridiculous.
You are taking a zero-sum approach to this argument. If developers have more to think about, everything suffers. Hell, if the developers decide to make an additional hour of the game, by your logic, every existing bit of the game is going to be less optimized and "Shit" right?
AAA game developers are good at what they do, and deal with a lot of complexity, a lot more than worrying about very similar platforms to optimize for. Give them some credit to be able to adapt and appropriately approach situations.
You can't substract all kinds of context and say that "the lower the price point, the higher the sales". That's true if you don't have other external factors messing with your equation. If the PS4 is 250$ and PS4k is 500$, sales of the PS4 will grow. It's so cheap that many people won't care there is a better system. But if the PS4 is 300$ and PS4k is 400$, a lot of people is going to go to the PS4k. It's obviously going to cannibalize PS4 sales. It does everything the PS4 does but better. 100% more teraflops for only 33% extra $.
And I don't think PS4k is going to be "just another SKU to optimize for". For some developers it will be like that. But for others, PS4k it's going to be the target they design they games for, and the version they are going to show at shows like E3, so they will spend extra time in that version. And then, the og PS4 is going to be "just another sku to optimize for". They will lower the res, remove detail here and there, less af, less everything and there you go, here is your crappy port of a PS4k game.
I know saying this will happen day one is crazy, I would never say that. But it's going to happen, progressively. You are going to see worse and worse PS4 versions of games as time goes by.
As you say, no one thinks the existence of Xbox One has made PS4 shit. But we have seen things. We have seen games that run exactly the same in both (parity for Fifa or NFS 2013). And we have seen how, lately, developers have started putting more effort in PS4 versions of the games, that now most of them run higher res and more stable frame rate in the PS4.
Going back to the Witcher 3. Before launch, they put all of their effort in the XBO version. So it had higher frame rate at launch. Then, they focused on the PS4 version, and with the latest patch it performs better than the XBO version.
That's the difference between developers targetting your system or developers targeting other similar system. And the difference in power between PS4 and PS4k is 2.5 times that of the PS4 vs XBO. The architecture it's the same, but every hour they spend optimizing the PS4k version is one less hour they spend optimizing og PS4 version.
And I give developers a lot of credit. They spend thousands of hours working so we can have 10-20 hours of fun (or hundreds in games like BF4) but they are humans and their time isn't infinite. If they do a game for the PS4k, you can't expect them to start from the ground up to make a PS4 version. They will just use their base version and remove things until it works on the PS4. And that will never look half as good as if they developed the game targeting og PS4 from the beginning.