I spend 8-10 hours interacting on my rMBP every day, using fingers/touch/gestures probably ~75% of total time. It's called the Magic Trackpad. Better Touch Tool extends that functionality even further.
With arms extended forward, fingers naturally point down, right where the trackpad is located. To touch the screen, I'd have to hold arm up and extend fingers straight forward. I know very few people who draw that way, it's not natural nor organic nor comfortable, whichever adjective you'd like.
Most artists I know in just about any medium draw with their hand curved downward, just like writing. Wacom knows this very well and their products are designed with that in mind.
The poor ergonomics, increased costs, etc. have all been pointed out already. Sure, MS has some slick marketing, but do they have sales to back it up? The surface is tanking badly already. HP has been making touch-screen AIOs for a while and I could be wrong, but I don't think they've broken any records either.
After the first 10 minutes of Minority Report wow factor wears off, a touch screen iMac just doesn't make any sense for several reasons.
And OP, if you've been waiting for a couple years, why haven't you bought one of
these already? Cost factor?