Well they seem to do so on PCs which have more regular and smaller updates?
I consider the idea of foward/back copatible progressively upgrading consoles great and totally plausible but there is more to mere flops and architecture to executing it right.
My stance is, the way this rumors are painting the picture, devs had little head up for this. PC games are developed from the ground up with hardware compatibility in mind, they run at a higher level, through windows, dx, and the video card's drivers, and even then fuckups are not uncommon.
I bet console engines and projects lack a lot of the infrastructure required to be able to just easily improve gfx quality substantially. Sure things like resolution and draw distance are just a variable they can modify right? Until they do and a 100 bugs pop out of nowhere. Oh it seems this gameplay designer coded this script on this level assuming this objext would be loaded into view at this distance. Oh, this system bases its calculations on specific pixel numbers over here, but later it would use the relative screen coords. Gotta rewrite that... game dev is messy, and deadlines and crunch leaves a dirty trail of shortcuts and hacks all over the place. Even multiplats released on consoles, run in a branch of the engine that rids itself of a lot of the expensive infrastructure of the pc version. That has to be put back in now. This are simple changes from an enginerimg pov, certainly, but on a large AAA game, full of features and content, and a large codevase, they take weeks, if not months. Most devs will half ass it initially. And MS will be up there trying to sell something as big as the bone and louder, costing 600 bucks, that is suposedly a beast, and all they have to show is the same games of the regular bone at slightly higher res and better fps.
The avarge gamer will see no difderence. Digital foundry is not a console seller.
Lets even ignore the technicalities, and suppose developers can just mark a few checkboxes in their engine as if it were a pc gane and out is the Kaio Ken and One Two versions.
What would the graphical enhancements be? Longer draw distance, better SSAO, aniso, texture streaming, higher-res post processing... basically the difference between a game in ps4 and a pc on Ultra everything. I can sure spot the difderence (with some effort actually, this day and age) but do you think the avarage joe gamer sees any difderence at all.
Lets get our collective heads out of our own niche beyond3d-member digitalfoundry-reading siggraph-presentarion-watching graphics-enthusiast asses and think of the large mass of consumers. You can grab a FAT majority of them, show a game in ps4 and pc ultra 4k side by side, and they will still tell you they look the same thing. They really don't care. They just wanna play the game.
So, do I think there is no place for a premium machine? No, I think there is. Even if a consumer is a layman, get a salesman to tell them X is MOAR BETAR than Z, those with enough money will buy into it. But there is a sweet-spot on how much more expensive that premium version can be before it becomes too niche. And the more niche, the less devs will care, thus their improvementes staying less substantial, making it even more niche in a vicious cycle.
The only reason I saw ps4 kaio ken as possibly working commerecially, was if it downplayed the improved graphics, and really just sold it as a ps4 that can play 4k movies, has improved media features x y and z, and oh, by the way, your ps4 games will play at 4K* too (*:1080+ upscaled with 4k hud). I know people here had much more ambitious speculations for the thing, but I was personally very sceptical of the marketability of that kind of thing, and of the willingness of devs to put the effort in to achieve them. As for Xbone two, I think 5-6Tf is bound to go mostly to waste.