Just a thought while we all wait, I happened to see the fabled Dell XPS 13/15 in a tech store the other day, and damn that thing looked cheap. Not being an Apple fanboy here, I actually approached it with a very open mind and I was astonished at how plasticky and edgy the thing is. Because of the small bezel the form factor is amazing, but the bezel that it does have is beyond ugly. The whole thing is plastic, sticks out terribly and overall looks like a super cheap laptop.
If I had to choose between a 13'' rMBP and the Dell, money and OS not being a consideration, don't think the XPS would get my pick. It's a very hyped up machine but it's not even the nicest looking Windows laptop. The Surface Book looks a hell of a lot nicer even with that hinge.
I think we'd all love a 13'' thinner MBP with the performance of a $3000 gaming desktop, but I guess this is something we'll likely never get.
And as much as Apple annoy us all with the lack of hardware updates, when it comes to the perfect laptop, Apple's design and OS reliability are pretty much unparalleled.
P.S. To illustrate my point, here's an image to show what I mean with the Dell: