Oh. You mean the Mohn scene. Okay yeah. Lusamine definitely had more satisfyin closure in USUM than in Sun and Moon but I still think USUM really dumbed down the story and certain characters like Lusamine which in turn shorten Nebby's story and kind of made Lillie and Gladion appeared to be spoiled brats. I still think the original Sun and Moon's story had far better closure altogether than USUM's. Sorry but there is no way you can convince me that USUM's story is better than Sun and Moon's. So much of the major changes in USUM's story just feel so forced.
I can argue why I disagree with you about what they done with Lillie and Gladion in USUM.
While the Ultra Do nothing squad exist for building up to Necrozma (Who has enough of a back story to grasp and has done a few things on screen villain worthy.)
A few other things in USUM make more sense than you are letting on.
Lillie and Gladion were still abused in the USUM story. It is VERY clear if you haven't figured it out!
Gladion found potential in Type:Null that Lusamine sure as hell didn't so he took Type:Null and ran!
Lillie couldn't stand Cosmog's suffering, took it and ran! (Not wanting to see pokemon get hurt in those levels IS Lillies character!)
Lusamine being a stubborn,prideful person; refuses to listen to a couple children who to her is as she said it. "Would be much happier if they listen to the adults around them!"
Again... to me at least, USUM didn't dumb her down. It removed the Nihilego venom that turns her into the most unfiltered, most extreme, most WARPED version of her seen in the last game and showed us what was actually her. (Still a villain and a crazy for that matter! But the reasons behind are much more explained INSTEAD of being put in post game dialogue that a lot of people missed.)
Leaving Lusamine opened to redemption in S/M pissed a lot of people off so Game Freak done what they did in both the Anime and USUM. That being making her less villainous/ explain why she became a villain in USUM or with the Anime not even evil... though once Nihilego venom became a thing, she still said nasty things to her kids and tried to kill a couple times.
Her talking about wanting to stop Necrozma more stemmed from her wanting to vent out on Ultra Space and look for Mohn! Causing her to do so many immoral actions to achieve it. (Makes her more human and complex as a villain, instead of the one dimensional villain a lot of people see her as!)
Gladion's own words in USUM if Lusamine goes into that wormhole... there is a REAL chance she would only succeed in disappearing as well which would result in more trauma for the Aether kids.
If you must know about Nebby's story, it's conclusion simply takes place in another place! Even if Lillies reason for handing it over to you is to help Necrozma, it is a lot less forced to Lillies she isn't a trainer excuse. (Mahalo Trial is the new place to catch it!)
Heck, the only reason people complain about USUM so much is because Lillie isn't the main focus of that story... you are! (That does translate to Lillies story being weaker I agree, only by the default of her NOT being a focus. But saying it is ruined is pushing it.)
That and the Exeggutor island scene was only there to tell you that Lusamine wasn't always a psychotic monster (Which a lot of people missed anyways or misinterpreted it as something much worse.) As well as Lillie confessing her feelings for you.
USUM humanizing Lusamine in addition to Lillies moving her confession scene at the end of Rainbow Rocket made that scene unneeded.
Lillie gets more closure as well in USUM, getting to stay on Alola with all the friends she has made rather than go into a strange region by herself!
In USUM Lusamine changed for the better and rightfully so is staying out of her kids lives unless they actually need her presence. (Correcting her behavior on her own volition.)
While Gladion... even though he could have been used better in Rainbow Rocket, got to go explore the world with his Silvally. Getting to be his own! (He even comes back to Alola!)