Look at a series like GeGeGe no Kitaro, which has had an anime adaptation in every decade since its debut, each with the same basic premise, cast of characters, and character designs, and many specific plots recycled from adaptation to adaptation as well. The more episodic nature of the series allows each iteration to tell new stories, but it's not like the manga's still running, it's not like the franchise is churning out radically new or unique storylines.
I feel like Sailor Moon will also be a series that has a new adaptation each decade: we got the original anime in the '90s, the live action series in the '00s, and Crystal should have been limited to the '10s but clearly its production has been bungled at every turn, causing it to be dragged out into the '20s. As a result, I don't think we'll get a new TV adaptation until the '30s (to coincide with the franchise's 40th anniversary - again, I expect it will be CG animation because that's just the trend animation in general is moving towards), but I do think that perhaps shortly after Crystal wraps up (hopefully by mid-decade) we'll get the obligatory live action film adaptation. I also think that's probably when we'll finally get a fully colorized release of the manga (since there really aren't any other ways to milk that cash cow left, and the manga's previous major re-releases were to promote the latest TV adaptations).
I think there is a possibility (however slim) that Naoko will turn out a new side story in 2023 for the next Year of the Rabbit (as she did with Parallel Sailor Moon in 1999; 2011 saw the unfreezing of the franchise with the revised manga's getting translated in various languages, a new video game's being released in Italy, and I suspect the talks that resulted in Crystal started around this time as well). But even if Naoko never writes another SM story, that doesn't mean the franchise is stagnant: La Reconquista introduced a new character in Lemures Leader Barbar (and Petite Etrangere and Amor Eternal had fairly unique takes on the Black Moon and Dream arcs), Sailor Moon Drops had at least one original story for the Super Sailor Moon event, and both Universal 4-D shows have had original stories (with the second being a bit more original). If they continue to do those Universal shows, I'm sure we'll keep getting new stories for them. If the musicals continue, I'm sure we'll get new stories there as well. I know the franchise may seem stuck in a rut at the moment because of how protracted Crystal has been and because there's been a series of back-to-back Dark Kingdom arc musicals with NogiMyu 2018, NogiMyu 2019, The Super Live, and Shining Moon Tokyo (and probably this year's upcoming ice skating show, too), but after this year's Princess Kaguya's Lover musical, they'll have pretty much exhausted the manga (unless they do a Sailor V musical, which would be awesome), and I think there's a really good chance next year's musical will be an original concept rather than yet another Dark Kingdom arc adaptation. Once Crystal ends, I know another anime or live action adaptation will be around the corner that will also feature plenty of original stories (episode-wise, film-wise, perhaps even arc-wise a la the Doom Tree.)
tl;dr Sailor Moon isn't dead just yet.
I feel like Sailor Moon will also be a series that has a new adaptation each decade: we got the original anime in the '90s, the live action series in the '00s, and Crystal should have been limited to the '10s but clearly its production has been bungled at every turn, causing it to be dragged out into the '20s. As a result, I don't think we'll get a new TV adaptation until the '30s (to coincide with the franchise's 40th anniversary - again, I expect it will be CG animation because that's just the trend animation in general is moving towards), but I do think that perhaps shortly after Crystal wraps up (hopefully by mid-decade) we'll get the obligatory live action film adaptation. I also think that's probably when we'll finally get a fully colorized release of the manga (since there really aren't any other ways to milk that cash cow left, and the manga's previous major re-releases were to promote the latest TV adaptations).
I think there is a possibility (however slim) that Naoko will turn out a new side story in 2023 for the next Year of the Rabbit (as she did with Parallel Sailor Moon in 1999; 2011 saw the unfreezing of the franchise with the revised manga's getting translated in various languages, a new video game's being released in Italy, and I suspect the talks that resulted in Crystal started around this time as well). But even if Naoko never writes another SM story, that doesn't mean the franchise is stagnant: La Reconquista introduced a new character in Lemures Leader Barbar (and Petite Etrangere and Amor Eternal had fairly unique takes on the Black Moon and Dream arcs), Sailor Moon Drops had at least one original story for the Super Sailor Moon event, and both Universal 4-D shows have had original stories (with the second being a bit more original). If they continue to do those Universal shows, I'm sure we'll keep getting new stories for them. If the musicals continue, I'm sure we'll get new stories there as well. I know the franchise may seem stuck in a rut at the moment because of how protracted Crystal has been and because there's been a series of back-to-back Dark Kingdom arc musicals with NogiMyu 2018, NogiMyu 2019, The Super Live, and Shining Moon Tokyo (and probably this year's upcoming ice skating show, too), but after this year's Princess Kaguya's Lover musical, they'll have pretty much exhausted the manga (unless they do a Sailor V musical, which would be awesome), and I think there's a really good chance next year's musical will be an original concept rather than yet another Dark Kingdom arc adaptation. Once Crystal ends, I know another anime or live action adaptation will be around the corner that will also feature plenty of original stories (episode-wise, film-wise, perhaps even arc-wise a la the Doom Tree.)
tl;dr Sailor Moon isn't dead just yet.
