I’ve used Angular, Dart, Backbone, Ember, Elm, React, Vue, Svelte, and a few others I can’t remember anymore. All in production systems, not demo projects. Also some of the “build it once” platforms like Meteor.

They’re all cool until you have to actually maintain them. My favorite part is having to build my data models twice, one for the producer and one for the consumer. That’s totally never caused any headaches or slowed anyone down at all.

Unless your new client is a hybrid/webview/electron application, it's a trap. APIs for web, for native and public API have different sets of constraints in terms of authentication, versioning, even features are probably gonna be different.

And it's not like building an API after you built web would be complicated. Unless you put your business logic right into controllers/handlers, making an API would be just making a new endpoint that calls to existing application services. It's not free, it may not be trivial, but like other comment said, if you can afford a new client, you probably can afford an API for it