There are two completely separate lines of Minecraft ("Java Edition" for PCs vs "Bedrock Edition" for PCs/game consoles/...), so they've done a rewrite, but not dared kill off the original product, because they aren't compatible.

Not just compatible, the game mechanics aren't even the same :(

I think the end of the line for Java Edition will come at some point, but I also think they know perfectly well that they cannot do that prematurely, and we're very much still in "premature" territory.

You know they have right? The Window 10/Xbox/Pocket Edition is a complete rewrite in C# afaik

C++, actually.