The main reason why Scrooge, the kids and Della being the series’ six main protagonists can work because Della is influential enough to be one of the series’ main protagonists (she’s the overarching protagonist of Season 1 and one of the two main protagonists of Season 2), the role of protagonist varies across the seasons (For example, Dewey was the main protagonist of season 1, while Della and Louie took an active role for that season 2 and while Huey, Webby and Scrooge took an active role for that in season 3. Scrooge was certainly an influence, however, he doesn’t become the most influential character until the later half of season 3, a hero can be a strong influence to be one of the main characters and a overarching protagonist can be a strong influence too, as WillieFiveNights pointed out, however, it is very important that the hero is both active and direct in order to have an impact on the story that we’re watching, and as such Dewey, Louie, Della, Webby, Huey and Scrooge are the main protagonists in their own seasons and they both have the biggest felt. Many can argue who’s more prominent, but in the end, Scrooge’s influence across the whole series is affiliated, hence why it is stated to be one of the six main protagonists as default) and every hero have their form relevancy, throughout the seasons, simple as that and there doesn’t have a sole main protagonist for a show like this because every season centered a different one, there was never necessarily one main protagonist compared to many other shows, so it wouldn’t be rocket science to say Scrooge is the series’ main protagonist.