Sorry, allow me to be clarify my previous comment. I am agreeing with you. Pre-made assets are designed to be placeholders and should be replaced before the final release of the project if the developer can afford custom art or can make the art themselves.
Small caveat to cover my butt and is not necessarily in response to any previous comment: Many developers on itch.io are unable to commision art or create art themselves. There is nothing wrong with indie game devs using pre-made assets in the final versions of their projects so long as they are used as the artist intended. As an artist, I love seeing the art I sell on itch.io in people's games because it's like giving life to something I created.