This is mostly for fun but....
Not sure how the Japanese box office works but I know most movies have to make at least 2x or more it's budget required to make another film. Only guessing here because I don't know Eternal's budget (and Im bored and like speculating just for fun but I'll be using USD instead of yen) but a
high-end anime episode costs around 300,000 USD to make. An anime without commercials is roughly 22 minutes in length and Eternal is 80 minutes meaning the first film could have a budget of 1.2 million dollars. The second film is about the same length and I will assume it also cost 1.2 million USD (but maybe even less if there is a drop in quality) with the total of both movies costing 2.4 million USD together at best.
If they have already allegedly made $900,000 USD in the Japanese market from the first movie, considering the Outers are fan favorets and the Eternal Sailor Senshi have never been seen by many, I will assume Part II will pull in just as much if not more money. I suspect they will gain at least half of their budget from the total revenue of both films combined which they will probably do. Advertising for films costs money as well but Eternal hasn't done much to be honest which likely brought some of the costs down in that department, so I assume they planned this from a financial perspective ahead of time to save money long ago.
The money they made from this film while not impressive is still okay, not sure of the merchandise sales over there in Japan but that will add to the revenue as well. However, the meat and potatoes lie in the supposed deal with Netflix and international distribution which I strongly believe (hope) is going to topple the budget for the Eternal movies combined. The international fandom is Sailor Moon's saving grace and that includes Eternal viewership, proplica sales, and other merchandise such as BluRay and DVD, music videos, etc.
While DoReMi is doing a little better than Sailor Moon in Japanese, it would most likely be crushed by Sailor Moon outside of Japan in the international market. The chances of SM earning its revenue back has likely went up now and with the pandemic keeping more people in, all they have to do is watch TV/stream which they can do while working from home, and much more.
Note from Hollywood Reporter:
Although anime features like Ghost in the Shell or Hayao Miyazaki productions have grabbed the headlines over the years, domestic TV series are the backbone of the industry, bringing in more than 10 times the annual revenue of theatrical releases, which amounts to more than $5 billion.
The streaming giant followed that upon its third-quarter earnings call with talk of a content budget of $8 billion for next year. A sizable chunk of that is expected to go to anime series, though not all of them will be produced in Japan, as Netflix aims for half of its content to be original by the end of 2018. "We've more than 30 original anime projects in various states of production these days," Netflix chief content officer Ted Sarandos said on the call.
I'm pretty sure that has gone up by now. I do believe we will get a StarS movie regardless because of Eternal exceeding the budget but now I'm more curious about the possibility of getting an entirely new Sailor Moon series in the future under Netflix if the Eternal and StarS movies prove to be successful. After all, Sailor Moon isn't the first Toei property on the streaming service and while previous iterations of beloved anime have been panned by some, things are starting to look better since Netflix is a financial powerhouse, and Yu Yu Hakusho, is getting a live-action treatment and the creator's wife is non-other than Queen Naoko. If both of their series are being aired and handled by Netflix, that must mean they have developed some comfortability with the Netflix and contract negotiations and usage of characters. Of course, I'm counting my eggs before they hatch but I believe we may get something new dealing with Sailor Moon in the oncoming years from Netflix. Whether it be live-action or a whole new anime series is up in the air but we do know Netflix is willing to drop a high number of coins for anime it deems profitable and if they have an interest in SM, that's a good sign for her future (at the moment).
Shine Pandemic Illusion!!