The panel discussion I linked to was really intersting. I believe they said that Searge (who was on the panel) is the leader of the add-ons effort.

They did say that Java MC isn't going anywhere.

Their stated intent is to make add-ons/plugins as powerful as they can, but there is a limitation in that they want them to be able to work in environments like iOS. Add-ons sre no problem: they're just json data files. They think they'll be able use use C# plugins on iOS, but I'm quite interested to see how they pull that one off.

Regardless, there are 30-40 million players of MCPE who have no modding at all today. This new system is going to be a big step up. even if it is entirely different from the way Java MC is extended.