You don’t avoid using JS at all but only use it where it benefits the user. For example, the users benefit from having those forms having nice validation, dynamic lookups, etc. but they don’t benefit from using a vDOM on top of the far more efficient browser DOM or loading an entire general purpose templating framework instead of just using the DOM to update an existing form.

Those are things which developers do because they think it’s easier than learning the web standards or looks better on their résumé and as long as you avoid measuring actual performance you can sustain these beliefs for a long time even while your users are seeing performance below the level of a 2000s Rails app with worse error handling.