Guttenberg and the ACF blocks builder is great for editing content. But ACF is not only used by developers for the content of a post but for the settings as well. For instance, you could have a related post meta box created with ACF and populating this box is part of the editing process. This is not going to be replaced by a block in the Guttenberg editor. Meta boxes are still very needed in order to allow more granular settings, taxonomies, relationships... And it's a long-term need.