Guys, be gentle with me, this is the first time I have ever done something like this, but seeing the possible theories of people to this chapter I decided I wanted to put my two cents into this.
Dandadan the story that you are, I’m not usually a manga reader, in fact, I don't like following current stories, it makes me jittery.
However, I am passionate about storytelling, and I like to watch details of a story and try to see what reasons the author may have for putting them there.
At first, I was a bit apprehensive at seeing Momo lose her memories, there had to be a consequence to her losing so much time before doing the ritual, but memory loss is such a hard trope to write, it’s very tricky and Tatsu is a braver writer than I am for using it.
But then it occurred to me a possible reason for it, and here is my theory:
The memory loss is set up for the climax of the Kozuka Knives arc.
“But isn't that arc done already?" Perhaps, but I have a few reasons to believe we are still in the middle of it so please hear me out for a moment.
I do not think stopping Murakami and co is enough to stop the Kozuka minions, he is not the true mastermind behind the attacks and there are still many loose ends around this particular arc. That and the characters outright theorize that the attacks will keep coming.
Also, there was no final celebratory dinner, and those are important to finish an arc in this manga.
And there has been a particular thing that has been bothering me for some time now. Something Kinta mentions in chapter 200.
The man has a whole speech about how the strength of their bonds could defeat any enemy, especially mooks with no sense of teamwork.
And he is completely right, Kinta is many things but he sometimes has one of the most sensible insights of the gang.
But a thought kept gnawing at me “Why would Tatsu feel the need to point this out?” In a limited medium like manga, why did the author take that time just to tell the readers just how important their bonds would be to counter the Kozuka minions? Why put so much emphasis on this?
Another thing I thought about was how Tatsu was going to raise the stakes, if the Kozuka knives arc is still ongoing then he would need to find a way for the gang to really struggle against their foes.
And Momo losing her memoriest? That would be a perfect way to make the current enemies a threat to the kids.
The gang without Momo loses not only their best strategist, their most flexible fighter and arguably the strongest asset of the team but also one of the two hearts this group has.
Losing her like this is a huge blow to the gang, perhaps big enough to shake the entire group. Just imagine how people like Aira, Jiji, Vamola and Okarun will take this news. I don't know what kind of shenanigans this plot will bring about but I expect a big imbalance in the group without Momo.
Especially because this is a battle shounen, and, as I said before, Momo is the best strategist this team has, it’s insane how many of her crazy plans have saved everyone time and time again, and no other character does it quite like she does. I believe this is the perfect storm for the Kozuka knives arc to reach a big climax.
Let me run a bit more with my fantasy here, just imagine, at the eleventh hour when the group is battered and hurting, who comes to save the day but Momo Ayase, THE Momo Ayase we know and love, fully restored and coming like a tornado to save her loved ones, snatching victory from the jaws of defeat. Kinda like how Okarun came to save the day in Space Globalists.
Granted, this is all just one silly theory of mine, there are near infinite ways for this plot to go, and it's damn near too soon to theorize anything concrete, much less knowing how crazy this series gets. But, hey I just wanted to air my thoughts out a bit.
Also, as a point aside, I don't think this story is near its end, if anything it could be reaching a halfway point, characters like Zuma and especially Kouki were just introduced, and plotlines like the club and Saint Germain are just in their infancy. Guys, we don't even know what the heck the Dandadan is yet.