The tight coupling removes a lot of scalability possibilities though. I understand the higher productivity argument, but wouldn't it be time consuming on the long run to lock yourself in an environment where you would have to go back and build an API if you want to extend your web app to mobile for example?

It depends on the needs of the mobile version. Often, a well designed responsive version of the same web app is enough. Even if it's a mobile native app, you might still be able to reuse some/most of the screens with the same html in a webview.

Even if you really needed it to be an API, you can always make the server side return either html or json depending on some request header. There's also a chance that the mobile version will be different enough from the desktop that it warrants new, custom built endpoints anyway.