Ragebait thumbnail but hear me out
Now hold up. The thirsty lime is right that Airakun not only will happen...but also that it NEEDS to happen at this point... That said, when it happens, it will not mean that Momokarun is done.
I believe Momo and Okarun will end up being together, however, the only thing that would make sense at this point of the story is that the tables turn and now its Momo who has to go after Okarun.
Our boy did everything he could and still it wasn't enough. Airakun HAS to happen to justify the regression that happened in the romance plot. Tatsu is trying to justify why Momo and Okarun decided to be with others rather than immediately being lovers to make the story more rich. The story is not about Momo and Okarun loving each other anymore, is about them maturing and learning how to understand their feelings and not end up together due to the supernatural, but rather, by choice.
I believe when Airakun happens, Momo will somehow get her memory back but it will be too late. Okarun will genuinely try to give it a go (and make Momo tremendously jealous not on purpose) and Momo, with her memory back, vulnerable, sad to have treated Okarun so dispondent, will try to get him back (and perhaps fall back on Jiji for support that will inevitably explore romance due to her heartbreak).
Airakun happening by Okarun's choice will show Okarun is not going to simp for Momo forever since that isn't love, its obsession, and will see that Aira is a person that treats him well and will make his best to attempt to fall for her. This is the character growth Okarun needs, and he NEEDS to reject Momo once her memory is back since he has to respect his new relationship with Aira (but of course will suffer in the process).
For his growth arc to be complete, Okarun needs to drive home the point of what hurts the most: "Without the Aliens and the ghosts, you would not like me. I tried and it wasn't enough for you, to the point you naturally went with someone else. I know that without all the supernatural events, you would not choose me, and I am done wanting to be with someone who does not want to be with the simple me".
Momo on the other hand, needs to explore a path in which she goes to understand her love for Okarun, but also understand and respect his new relationship (peeling from her toxic jealousy trope and respecting Okarun's autonomy and decision, which after the whole Amnesia arc would be a gut punch for her given how hard Okarun tried). Whether she attempts to be with Jiji or not I think is secondary but benefitial to her growth if she tries it.
Once they have explored that dynamic and some fun supernatural shenanigans happen in-between, I believe they will come back to each other but not in a trope destined way, more like in a mature way out of their own volition and having outgrown their insecurities and issues.
...Or I am wrong and Tatsu saw how much $ Rent a Girlfriend makes and decided to go for the paper 🙃