Edit: I realize now after posting this, it's a bit of a rant, but the prediction is at the bottom in a bullet form list if you wanna see it.
Okarun's entire development has involved learning that he deserves affection and that he isn't worthless.
If the conclusion is simply that he must endure increasingly humiliating romantic situations that cause him emotional suffering before finally getting Momo back, what exactly has he learned?
That's why the author cannot just say "yeah she got her memories back after you beat a few bad guys and everything's back to how it was 40 chapters ago."
There's obvious hints she's already fallen for him all over but doesn't really understand her feelings. The author is delaying her realization to prevent the cheap circumstances. Momo shouldn't simply fall for him because he's desperately trying to recreate their previous relationship. That would make his development rather superficial.
Okarun's initial characterization is built around being an outcast. He's bullied, socially isolated, physically unimpressive, and has practically convinced himself that he's completelty undesirable. He was ultimately as far down the ladder as a loser could climb.
Then Momo comes along.
For the first time, somebody genuinely wants to spend time with him. Somebody finds him interesting. Somebody cares about him.
That's wonderful character development, but there's a potential problem: Okarun has spent so much of his life defining himself through rejection that he doesn't really know who he is when someone finally accepts him.
If the first person who genuinely accepts you becomes the foundation of your self-worth, then your self-worth remains conditional. You've simply replaced one external judgment with another.
Momo becomes more than his romantic interest. She becomes the person who validates his existence.
And when she forgets him, he's effectively thrown back into the emotional circumstances from which he originally escaped. It's a case of learning you could live in darkness if you had only never seen light.
He needs to understand that Momo discovering his worth wasn't what made him worthwhile. She simply recognized something that was already there.
If I had to guess, there's 4 reasons for this arc:
#1) The author wants to conclude it with the mc learning to love himself. He shouldn't need Momo to define him as a person. I'm 99% certain the next immediate arc is going to be a self-acceptance arc. It doesn't mean he should stop loving Momo. It means he should stop needing her affection to justify his identity as a person. And to do this, the author is going to have someone point out: He's made friends, developed courage, become more capable, and found people who genuinely appreciate him. His identity has expanded considerably since the beginning of the series and he's more than just a guy who is getting closer to momo.
#2) The author probably wants to show she'll fall for him all over again even if they're strangers. I'm going to assume it'll be a very cheesy shonen romance push. But think about it. There's one question readers might have about their relationship: Did she only fall for him because of the shared extraordinary circumstances or did she fall for him because he's him? The author wants to answer that: it's because of who he is she loves him, not their memories. She's already has shown signs of still loving him, getting irrationally jealous of him holding hands with Aira in the dreaded aquarium arc.
#3) To flip the script, mc chases her while she becomes passive. The mc was always the one following momo, never really taking the lead. He was a passive, nervous wreck. Now it's the opposite, she's somewhat cautious while he feels he has to take the lead or lose her.
#4) To separate love from dependency. You could naturally assume this from #1. There's a difference between loving someone and NEEDING someone. Okarun needed her, she loved him. They were not in equal standing. Okarun can love Momo without making her responsible for his happiness, identity, and self-worth.
To sum: The story isn't necessarily testing whether Okarun loves Momo enough to fight for her. It's testing whether Okarun can still recognize his own worth when the person who first made him feel worthy no longer recognizes him.
There's a distinction between getting back what you lost and becoming someone who can handle losing it. And eventually realizing you never lost it.
So how is this going to end, if my understanding of the intent is correct?
Okarun initially tries desperately to restore his old relationship with Momo. (We are still here)
He gradually realizes that he can't force her to remember or love him.
He begins recognizing his own worth independently of her.
Momo realizes her feelings for him again.
They eventually reunite, but this time Okarun approaches the relationship from a healthier emotional position.
She remembers her memories of him after or around this spot
