I don't think this approach will ever come close to replacing wordpress. The only benefit I can see is it being decoupled and lighter in general. You can already pull data from the wp database through the WP API and I am doing just that with a new app I'm developing.
The wordpress ecosytem - with all those themes and plugins is just too strong, plugins allow you to plug and play alot of complex functionality that you would have to develop for yourself if you were using a headless cms.
I can see the benefit to some use cases (eg. when you want to structure your data the way you like etc.) but for the average person, wordpress is still the way to go.
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