I mean, you're right..
Minecraft Windows 10 Edition is basically the mobile version of the game Minecraft Pocket Edition but for Windows 10, meaning you'll be able to play it on both Windows 10 PCs and mobile devices. Also, you'll be able to play it with an Xbox controller, keyboard and mouse, touch controls, or some combination of these, which introduces some interesting new ways to create. You'll be able to play it online with Pocket Edition players on Android or iOS devices, and you can find people to play with via Microsoft's online service Xbox Live. It's more likely turning into a cross platform game.
Also we can not deny that C++ compiler are faster than the Java, so that would be a step forward into optimizing the game yes.
And you're right again, Modding in Java is easier, that's actually the same reason why Notch has chosen Java, because he was more productive and was coding at ease at the time when Minecraft came out.
I honestly think that moving to C++ is more of a benefit than a loss, as Modding can always be done. Nothing can stop a programmer when he knows what he wants from his code.