The thing is, it's competition between Microsoft and the Minecraft community. Sure, Microsoft owns both versions of Minecraft, but they don't own the community. The community decides what it likes most.
Microsoft wants the unified version of Minecraft to be predominant over the Java Edition.
They can't just immediately shut down the Java Edition and force people to move over the new unified version. They'll want to make the unified version of Minecraft be more feature-rich and "better" than Java Edition to the point in which in the community will naturally want to switch over.
My prediction is: with the pace of updates being compared with the two versions, the unified Minecraft version will eventually outpace Java Edition in terms of features. Before then, the unified version of Minecraft should have its modding API (which I believe is called the Plugin API) finished, a possible Mac/Linux version, and official dedicated server software.
Right now they're "embracing" the Java Edition, saying they'll keep supporting it.
Then they'll "extend" the unified version of Minecraft to the point where Java can't keep pace.
Finally, the community will naturally make the switch over to the unified version since the Java Edition will have become too "sluggish" when compared. By then, the Java Edition will be considered "extinguished"
Plus, there's a reason why Mojang gave out free Minecraft Windows 10 Edition to all Java Edition players.