Your opinion seems to be premised on the assumption that Furiosa would be part of the next Mad Max film. I don't think that would be the case. Max left the citadel. The final scene where Furiosa looked at Max disappearing into the crowd meant that their story together was over, and that Max was leaving for new adventures. If the sequel doesn't include Furiosa or the citadel, which I doubt it would, then it doesn't take away anything from the hopeful ending of Fury Road.