See? This is the reason I can't see why Sailor Moon wouldn't be able to get new animated content unless something between the original creator and Toei was going on behind the scenes.
Saint Seiya is literally a nobody in Japan and even though Sailor Moon is niched now, its popularity actually covers places where Saint Seiya never even dreamt of being a thing, like North America and Eastern Europe. It even has a history of box office bombing in Japan, just like Eternal. Which I think only applies to Part 2 in Eternal's case, because at the very least Part I went to the top 10 and I doubt Saint Seiya's 2004 entry did so.
Well, considering the amount of manga spin-offs Saint Seiya has and their sales (generally +20k sales in the first weeks), despite being a niche I wouldn't call it a nobody in Japan.
The failure of the 2004 movie was because conceptually it wasn't Saint Seiya - there was only 1 full fight in the movie and all other fights were off-screen or cut short. Basically, for the majority of the movie characters just stared at the screen, contemplating their life
Over the recent years, the movie has sort of gotten a cult following from western fans but Japanese fans were very vocal about what the director did and how they didn't want this kind of direction for the future of the series - hence the reason for the bombing of the series. Even Kurumada was very vocal about his disappointment after both Toei and the director bamboozled his ideas for the movie to the point he went to TMS for a new SS anime.
Sailor Moon Eternal on the other hand - lots and lots of fanservice for the OG 90s anime fans - Tadano design, trailers with focus on the personalities of the girls, recycled footage, recycled theme songs from the 90s anime... yet Eternal is made for only one group of people - those who know the manga and are fans of it.
Even Crystal 1-3 had some work on it for atleast trying to attract new fans but not Eternal - no wonder people who didn't like the first movie didn't go to see the 2nd part despite having the Outers and the conclusion of the story (you would think people would be even more interested).
On top of that, a very bland and systematic direction to the movie - they could have meshed all 4 of the Inner stories to flow more fluidly, like introducing Minako's woes about being unable to transform right at the start or Makoto meeting and engaging with Hawk's Eye very early on too to set a strong footing in the writing but no - had to follow the manga to a T.
The only fun Kon seemed to allow herself was to put a nod to Saint Seiya via the Outers' roll call referencing Pegasus Fantasy (just like she put a reference to Soldier Dream via the Inners' dash in the SMC 3 OP).
Cosmos seems to be following the same approach by baiting long-time fans who forgot about Sailor Moon via the trailers focusing on UsaSei and Galaxia's menace mainly...
I have a feeling history gonna repeat itself.