Just because it doesn't out sell it doesn't make it a threat. Here's my experience with a Surface. I used to have an iPhone, an iPad and a MacBook Pro, the surface replaced it by being a tablet whenever I want it(and yes I am aware people say windows is horrible for touch. They have a tablet mode that's just like iOS and a desktop mode. I choose to use Desktop mode and have no problem at all using just touch on it. I throw on the Type cover, which is nothing like the horrible keyboard attachment Apple has for the iPad. It feels better than the new keyboards on the MacBook Pros and IMO the trackpad is now on par with Apple's. It's nothing like the old Track Pads I promise. Then if I want I can dock it with a Large monitor and keyboard and mouse and basically have a desktop as well. and at anytime I can also hook up any thing that uses USB. Before, I used to use my phone a lot when mobile around the house to browse things and now its reduced to pretty much just being something I use to call people or text.
iPads aren't even good at being tablets as long as they run iOS which is really a mobile phone OS proof being even Apple hasn't optimized it for tablets.
And once again here is the proof that it is able to replace all of these by a sub thread here on Mac Rumors which is very active and people are happy with their Surfaces.