I would personally stick with the 15" as well (probably), but around here the market for a 17" seems to be pretty large.That's what I was wondering: how many of the 15" folks would be getting a 17" if it were offered? Let's say, with 32 GB of RAM and a 4K screen?
I probably wouldn't, as enticing as it sounds, because it would cross the line into not being portable enough. 15" is a perfect balance between desktop replacement and portability.
A desktop replacement would be a nice option in this increasingly mobile world, but then that takes away some appeal from their brand new 5K Display.