I went to a different store at the weekend and had a closer look at another Surface Laptop. I have to say, my concerns were still there.
The fabric is strange, and apart from the issues of life and wear, this display machine had an even rougher edge than the first machine I looked at in a different store. The fabric was sharp all the way around the edge of the keyboard, and was even jagged in places. If they're going to glue fabric to the deck, it needs to be absolutely smooth and seamless.
My focus on this trip was to do more of a typing test, and while the keys themselves were good, the flex of the keyboard is just insane - the whole thing bounces, and when I pressed down hard it bent like crazy. There was a Surface Pro with detachable keyboard next to the laptop, and that keyboard was stiffer than the one on the Surface Laptop! It was seriously sub-standard. I'm surprised only a couple of reviews have picked up on the flex. There was also a Surface Book, and it was rock-solid, like my rMBP.
I haven't examined the iFixit teardown in detail, but I would be very surprised if the keyboard/deck was aluminium like the rest of the machine. It feels like it is plastic, to save costs, and they've hidden that with the fabric.
I wouldn't be surprised if this time next year MS starts offering a purely aluminium version. But certainly, I wouldn't be happy with the current model.