The "but" comes in when you talk about existing themes and plugins - some are much larger applications, and all are built in PHP. So to migrate, you'd need to reimplement both WordPress itself and various themes and plugins in Node.

Not to mention that you'd need to build traction with developers to keep supporting it. Not every PHP master is also a JS master, so finding buy-in from the existing community will be hard.

Also, remember that PHP comes standard on most shared hosting environments - which is where WordPress is most often installed. Node, however, does not. So once you have developers on board, you have to get hosts on board to help set up the application.