Ok let me clarify: the GPU is not all that matters. It can have different frequency, different amount of VRAM, etc. to reduce the power draw. And then there's thermal throttling. Those laptop cards eat up to 150W (just verified) of power, which is almost normal even for a desktop card. OK, if the laptop's power adapter can handle that, I understand. But the cooling! To dissipate that much temperature, you need a big ass fan. The card I'm using in my desktop is roughly 150W, and it has two big fans (twice as big as your default CPU fan) with 4 heat pipes, and they are just enough to cool it.
The heatsink is longer than the card itself!
This card weighs over a kilogram alone. It's over 5 cm thick. Oh, and did I tell you that the cooling solution is some patented super efficient Gigabyte stuff?
And don't tell me that Pascal has improved. The TDP is the same (well actually mine is 140W), and we don't have any new "magic" cooling method. So the cooling part has to be just as enormous as this one. And then it needs a separate power supply, which also adds to the space and weight. In the very best case, it will still be at least half of the size (and weight) of this graphics card, so we end up with a "mobile" graphics dock that is twice as heavy as the PF and (minimum) 5 times as thick. Is that really "mobile" enough for you to justify choosing not to accommodate other cards and not to allow it to be replaced?
Now to the "desktop" solution that you proposed... as I said, $300 is a tremendous amount of money. For that sum, I can build myself a little neat computer that has similar computing power to the Pyramid Flipper. So I'll have two computers instead of one. And one of them will have dedicated graphics.
Now, I want to propose my alternative:
Remember how I said that if you want full desktop performance, your "mobile" dock will be come "not-so-mobile", thick and bulky? Well, we can add like 1cm to each dimension of it, make it easy to open with screws, and ship it without a graphics card preinstalled. Let's take your $300 price and subtract the price of a GTX1060, which costs $200: we get a $100 unit that isn't much larger than the one you suggested, and that accepts most desktop cards, with an understandable size constraint. I think it brings worthy tradeoffs to both ideas for a compromise: 1cm in each direction isn't that much when we got to the point that it's 5cm thick - 5cm or 6cm doesn't matter that much anymore; and the limited space is a small tradeoff, but it's not terrible, since pretty much all cards are the same in size nowadays.