The new Windows 10 version is, for the moment, more tailored to people who are already within the PE ecosystem, whose kids already play it on their tablets and phones. Crossplay for that segment of the community is huge and shouldn't be understated; a lot of parents will be able to play with their kids easier now. If you're mostly playing LAN or within the house with your family, or even running just a small server (locally or on Realms) for immediate family and friends, then the Windows 10 version is going to be appealing to you. And for those who want that experience, that's awesome
As someone who primarily plays on the current PC version mainly in multiplayer and/or modded, the reasons for me to jump on this right now are much fewer and farther between. Other than perhaps moving to C++, the incentives aren't there. If they are able to bring it up to full parity with the Java version (and the community decides to bring their servers and mods up to support C++ as well), then a switch is perhaps more likely.
Right now, Pocket and Console Editions are not there yet, and hopefully the answer to that is not to bring the Windows 10/Pocket Edition into parity by adding in all the content currently available in the PC edition, but stripping out large multiplayer server support until they can get it figured out. Yes, I know Pocket Edition supports mods, too, but the modded Minecraft series that most kids are fans of aren't going to be recreateable experiences on PE because the mod library for PE is so much smaller right now in comparison.
In the worst case "doomsday" scenario (you know, the one people are worrying about in this thread), I'd imagine the PC community would continue to support the Java version until the Windows 10 version presents itself as a real equal to the current PC experience.
Now, if Microsoft wanted to REALLY play hardball, they could pull the ultimate heel move and just turn off the authentication servers for the PC version one day to try and force everyone onto the Windows 10 version ... but the blowback from that would be so huge that I think it'd cripple the game from a popularity standpoint. I can't see them doing that and I'm not worried that they'd attempt such a thing without Windows 10 version being at a true equal parity to PC edition because they'd be giving the big-name Let's Players/streamers on PC (who are in the majority, even with folks like Stampy who have made a living on the console edition) every incentive to walk away. Most play outside of straight vanilla now, and given the shift in what's popular in Minecraft on YouTube/Twitch currently, going back to straight vanilla isn't a good business choice since the market is flooded with so many straight vanilla LPs right now.
So, I guess a lot of the future of Minecraft depends upon how quickly Windows 10 version grows and advances and what decisions they make regarding support in the future. But I can't see them abandoning PC edition anytime soon, and if they choose to, the core of the PC community is probably not going to jump to Windows 10 version until there's a reason to.
