To be fair, Modern Warfare is not that demanding of a game, it uses a modified Quake engine which could run on a Dreamcast console. (A console that came before the Playstaion 2.)
John carmack is amazing at making game engines run incredibly tight and at a high framerate, that's where most of the praise came from with that engine and that translated over to Call of Duty.
Battlefield 3 however had map sizes and player counts reduced to fit on the Xbox 360 and PS3's anemic hardware.

That's the trick, trimming in the right spots to maintain the core gameplay.

But as dev after dev is stating, graphics are vitial for immersion - Crytek, Epic, Kojima Productions, Quantic Dream etc so mainting core gameplay may require graphics to be cutting edge or at least up to a very high standard, and it won't be trimming as much as it'll be complete amputation of features.

In short, it'll take so much effort and result in such a watered down product, they probably won't bother porting games to wii u. 

Graphics don't need to look like KIllzone Shadowfall to be immersive though.

A lot of devs are saying that graphics help to improve immersion. Reduce immersion, reduce overall effect of the game. 

Not necessarily; control can be far more immersive than graphics.

Someone did post that in a separate thread I believe.  I am of th4e belief that article is a ton of a wishful thinking that can be summed up as: "Don't worry. engines are MONSTEROUSLY scalable, so the Wii U will be getting everything about, because games will be as they are today".

Companies also can be more inclined for that "right spots" could be allocating developers to do cutting, particularly if they believe the platform in question isn't going to sell their title.  If the goal now is to not cut corners so it is real close to the PC version, then the Wii U will be out of luck.  Otherwise, why not just go ahead and do a really cut back version for the PS3 and 360, and merely add the Wii U in the mix also?  Do people seriously think that this argument about, "But, but companies don't want to miss out on sales of the established base of the Wii U, because it means money... but NAH the PS3 and 360 can't handle these games so the WIi U is the ONLY choice!"

Strawman - controls are independent of graphics and I never stated controls as being more or less important. Either way devs are saying that graphics are vital to immersion and the Wii U certainly won't be able to keep pace with the PS4 in that sense partially due to RAM. 

Devs don't decide these things, the player does.