Wii U uses a console architecture. XBO and PS4 are just PC's with a custom OS. PS4 and XBO have diminishing returns. Consoles traditionally because of how they were designed games got better looking over time as they optimized better. PS4 and XBO being a PC you see what they have to offer now.
X86 machines they have need just raw specs because X86. Wii U is custom in every way so that every part is made to work with another making so it doesn't need the raw specs PS4 and XBO need. Wii U is built as a console. Wii U still can't produce games as good looking as PS4 or XBO but it's not far behind. It's no slouch. Some games we see on PS4 and XBO can run on Wii U fine but the real difference is 1080P on PS4 and 720 on Wii U.
Everything has diminishing returns.
Even though ps4/xbone are basically off the shelf pc's with a netbook cpu crammed into an apu, they are still closed systems, as opposed to actual pc's that can have thousands of different hardware and driver combinations games have to be compatable with. Which means they also have a lot of room for optimization. Although, it doesnt look like any AAAAAAAAAAAAAAAAAAAAA publishers got time for dat.
Crap out games, match parity for legal platform holder bs reasons, dont take any advantadges of system specific features, and simply tell the dumbasses who buy your games that the missing features, broken games, and de-evolution of game design, are modern features that are actually BETTER than the obsolete game design of games that worked as recently as metroid prime, and market it until you brainwash them into believing it. Just like Activision says they do.
Not really seeing how this is different for consoles than PCs. "Console architecture" is a completely arbitrary term. The improvements over time happen everywhere. Skyrim looked a lot better than Oblivion because of all of the graphical techniques they learned throughout the last generation. It looked better on consoles and it looked better on PCs. In fact, Skyrim runs on my PC a lot better than Oblivion does.
If anything, I'd think it'd be the exact opposite. PS4 and Xbox are going to increase in potential MORE than the WiiU because they can largely share a knowledge base with PC developers and therefore more pioneering techniques will be discovered for them.
This isnt that likely of a scenario, because of the reasons I already stated, publishers have ZERO interest in actually forwarding the progress of game design and optimization. That requires talent, risk, and motivation. They are far happier to keep things exactly where they are, and use marketing to convince their brainwashed audience that crapping in their face is actually advancment, and anything else is backwards.
Aside from that, ps4/bone are in fact, closed systems, while the pc scene, is NOT. PC technology will continue to improve, utilizing the latest in its gpu/cpu technology (When its not a complete and absolute fraud, G*Glares at Nvidia*) however, these will NOT directly translate to ps4/xbone, as they will mantain the same processors for their entire lifespans, meaning THERE advancements will come from better utilization of THE SAME processors, in optimization, rather than advancements made possible by new hardware. In fact, the pc scene is ALREADY being held back by the underpowered ps4/xbone, as big name engine companies like epic already REMOVED their next gen lighting engines because ps4/xbone couldnt handle them, and they didnt want to be blacklisted by two huge partners making ps4/xbone look dated before they ever released.
Due to the nature of all consoles now having programmable pipelines, instead of Nintendo being stuck with fixed function pipelines, any new effects that can be approximated from pc to ps4/xbone, can in fact be approximated down to wii u, and even smart phones.
Thats the reason Nintendos games looks so damn good on wii u. Its not the instant dismissal favourite 'Art style'. Nintendo has INCREDIBLY high end systems they work with. Where do you think those ridiculously quality character renders and things they use for splash screens come from? These are things no ps4/xbone, or current home pc can do in real time. However, they take these advanced effects and approximate them into similar looking effects that CAN work on wii u.
How far thats going to go, depends on whos doing it, how good they are, and how much drive they have. Its becoming increasingly obvious, by the RIDICULOUS leaps wii u's been making already, that they have some seriously motivated people in that department. A human resource factor neither sony nor MS has in their corner (Ms never had them for gaming as msgamestudios has never been anything more than a transient quantity of bought 3rd parties, and sony is currently laying off all of their talent except santa monica and naughty dog). Which is why, Nintendo games get BETTER looking as they near release, and other studios get constantly whined at for 'downgradatons'.