I want to play with it, if and only if they add in keyboard and mouse support. From what I've been told, it's currently for controllers (I saw something that contradicts this). I've always thought it would be fun to explore MCPE more thoroughly if I had decent controls to do it with, because playing on a phone is just painful. I tried to run it on PC before through Android emulation but the graphics emulation was the hitch.
MCPE is pretty nice, and has a lot of features that the PC version simply doesn't. To be honest it comes across as being much more professional compared to MCPC. It's not because of a lack of developers on the PC version, because they keep overhauling that one significantly, just never really in the areas that matter most to players. I mean there's still no crafting reference, for crying out loud. We're beyond the point where you can guess how to craft things. The wiki is a requirement to play this game, which is a bit ridiculous. It wouldn't even be hard to add a basic in-game crafting guide, they just don't.
As for whether this will replace desktop Minecraft, I personally think the answer is clearly "yes". Not for a while, but definitely yes. They even said that MCPE is about to get a lot of new features as a result of this. They just have way too many versions of this game, and all of the others are written in C++. Maintaining one in an entirely different language means zero code is shared. Eventually I think they're only going to hang onto it for legacy reasons, and to ensure that people on other platforms will be able to continue playing it. It might still get bug fixes by whatever residual Java dev team members they keep on, but that's probably it.
Keep in mind that Java is a direct competitor to .NET. You might remember that one of the first things that happened after Microsoft's buyout was that Java was embedded into the launcher and completely hidden from the eyes and minds of players. It's still good for users too, but obviously wasn't a priority until then. They bought this game purely for promotional reasons, it's the only way to justify the price tag, and that's exactly where they've been directing their efforts. Windows Phone, tablets, Surface, Hololens, and now Windows 10. A while back I honestly thought that they might give it away with Windows 10 to encourage adoption, but that's obviously not going to be the case now. Hell, they're giving the entire OS away now, so that's incentive enough I guess.
I still want to try it, and I want to try modding it to run on older versions of Windows first and foremost (I'm not jumping to Win10 just for Minecraft), but we're going to have the Windows app store DRM to deal with on top of it, most likely. And we can only hope that they leave some debug symbols in the exe, or modding it will be incredibly hard.