That's a terrible idea, would probably be an expensive dock as well if its possible.

Instead it makes more sense to release a more powerful Switch 3 or 4 years from now.

It can very easily be that simple and no different from the existing set up with undock versus dock. Essentially CPU performance is fixed but resolution and fx is scaled. 

No, the dock doesn't give switch processing power. It gives it a power source so that the switch can run at a higher clock speed without running out of battery in 20 minutes.

Do you say this from a technical background, or from a perspective of "I can't think of a reason why it's not feasible, therefore it is"?

Because from a technical point of view, as I stated previously. It really isn't feasible. The presence of a USB-C port alone is enough to make this not feasible. But if you have reasons why you think transferring data from a processor to a co-processor over a bus with a bandwidth as limited as USB-C's is not only feasible but could net meaningful performance gains at an affordable price, well be my guest. I'm all ears.

My response wasn't about the technical possibility of a functioning SCD, it was about a hypethetical powerful dock automatically equating to portable games running worse. 

"docked games will run better but then you have to think, how will undocked games run :-/"

" The difference in hardware would eventually catch up to how the undocked switch plays games."

Well that's a valid point as well. Right now the difference between docked and undocked mode is really little more than screen resolution. You're not talking about higher resolution textures, more complicated 3D models, animation, rigging, or anything like that. Adding extra power to accomodate those kindsenhancements means building the engine to handle the swapping of those assets in real time, and that would mean there would be less system resources to make the handheld mode run as well.

But why would you swap out different assets? As I said earlier, there are many things the docked is already failing at, resolutions above 720p is one of them and anti aliasing is another. Most engines are already applying scalable vfx and LOD.

...because docked mode already renders in a higher resolution, scalable visual effects, and better level of detail? What's the point of the additional hardware if you're not going to use it other than to do slightly better enhancements to what it is already doing?

Super Mario Odyssey is 720p with jaggies in abundance. And Breadth of the Wild's (A Wii U game) is 900p with LOD that can still be scaled up. No doubt when Nintendo makes an Native zelda game, they'll be further away from a 1080p target whilst continuing to take shortcuts on LOD/VFX/Lighting as higher quality assets strain the hardware even more. Not to mention the potential for HDR/4k/Upscalling. 

But maybe it won't be of interest to you just as many people see PS4 Pro as pointless, but I doubt they'd have an issue selling a $149 SCD. 

The jaggies is because Nintendo is allergic to Anti-Aliasing. A technology that has existed for almost 20 years.
Partly that is due to the lack of hardware capability.