Unless you’re changing other parts, too. You will NEVER hit the :new action because you ALWAYS create the comment form in the context of a Post or another Comment. Like I said, you could add , except: [:new] to the route if it bothers you.
So in your example the forms already on the page in index view instead of a link to new comment? If yes whats wrong with a link to new comment instead of a text area and submit button already rendered?
If you do that, you need to pass the “parent” object to the new action. There’s nothing “wrong” with that. You probably need to define your own new_comment route if you want the URL to look nice and not have query parameters to define the parent object. (If you intend to eliminate threaded comments where the parent is another comment, you have just a little more work to do, but then you might not even need Comment to be polymorphic [if you can’t comment on more than one kind of model].)