In my case, I’d make a few calls to the API to gather data, do business logic, and send something to the API to save in the database. I quickly realized that this is not the way to do it, let PHP and the server handle all of the business logic, even dealing with other API’s, and only use JS as a UI layer. It can be a very robust UI layer which calls on the WordPress REST API for data, or other API’s for data, but no actual business logic.
We don’t foresee a near future where WordPress is built on node, at least not the .org version. Maybe if the stack for PHP did not evolve, but it has come leaps and bounds, making it easy nowadays to create and run on a great server that powers PHP applications. To build WordPress out again, powered by a node stack would just be a rebuild, and also would mean its not going to be as easy to configure, manipulate, etc.