> Whether or not you love or hate this model of development is irrelevant, because if at some point you need to render on a client other than a browser -- you have to start from the beginning and write an API now.

That's not true generally since many mobile apps are simply wrappers around a WebView. Rails Turbolinks basically has mobile shims that allows you to deploy to all platforms. Sure, if you can't use a Web View on your mobile app (idk, for performance reasons) that's something else. But that's not the rule.