If this really is a PS4k with upgraded rendering and not a PS5, I would expect that developers will still be using PS4 as their target during development, with a tiny amount of extra development to allow the game to run at a higher resolution/frame rate on PS4k. Sony's managed their consoles really well and I can't imagine they are going to allow developers to split their PS4 user base and make games that only target PS4k.

It's not a problem for all the different 3DS's, certainly not a problem for xbox 360 elite/Super elite, and Playstation slim range. So, why now?

Im totally fine with it. Its just like on PC games tend to run on a variety of specs and with the way GPUs and CPUs are going I cant see why they couldnt create games for the lower end model (PS4) and just improve the resolution and texutres for a PS4.5 or whatever. It works great for PCs and it would be fine for consoles too.