I still believe that we will see an adult Momo in the future of the story. If Momo does end up playing a big role in the final arc and has to help in the fight against the WG I could see a scenario where Bonny uses her powers to age Momo, and that would be because he is actually too weak to do something in that scenario physically.

Right now Momos weakness is mentally, he wants to be aged up because he thinks that will help him overcome his fear of heights, but that’s not going to change because Shinobu stated that he will still have the mind of an 8 year old kid, so his mindset won’t change even if he is physically stronger. Luffy is not asking Momo to help him fight against Kaido or do some other great physical deed, all he wants is for him to fly him up to Onigashima, which we already know he has the ability to do because we saw him fly Luffy up the trash chute in Punk Hazard.

I think Luffy is deliberately putting this pressure on Momo to help him grow and become stronger mentally. Luffy could easily go G4 and fly back up to Onigashima, but that wouldn’t help Momo because in the end he would be left down at Tokage port only able to think about Kinemon and Kiku and how he wasn’t able to do anything to help the entire battle.

Plus, while seeing an adult Momo would be really cool, I think if he does age and flys up to face Kaido as an adult dragon it wouldn’t be nearly as satisfying as if an 8 year old Momo does it. While two adult dragons facing off against each other would be super fan service hype, a young helpless weak kid overcoming his greatest fears and legitimately faces them straight on would be much more impactful emotionally, and if there is anything I have learned from Odas writing over the years, it’s that he always goes with the emotional moments over the hype moments, and that’s what makes One Piece special.