The hardcap of an ICO might not be a guarantee for the success of such project, but it is necessary and at least one of the criteria for solid and genuine projects to be able to deliver their promises to later.
Well, there is no guarantee for the success of the projects but it indicates that there is a high chance for the project to be successful. By reaching the hardcap, it means many investors join the project. It tells us that the project has something special to run and will lead to a good future.