We are never given any hint that Simpleman could reverse his powers. By the second simple-wave he launched, it had rendered everyone (with the exception of the kids and Simpleman himself) completely mush-brained. This also applies to Shadow Moth. If the kids weren't there, Simpleman would have gotten their miraculous, but then what? Give them to Shadow Moth? Shadow Moth was effected by Simpleman's powers and turned into an idiot same as the rest of them. He wouldn't be able to tell him where to give it to him. He would also have been too dumb to deakumatize Simpleman, meaning the entire population of the city would have been left in a state where none of them could conjugate, operate any form of machinery or do anything that a two-year old could do.
Not to mention, outside of the superheroes, imagine everyone who was hit as they were doing their job. Be it doctors, the water industry, power plants... if it weren't for Ladybug and Cat Noir, Paris would soon be unhospitable for anybody to be in.
When Simpleman used his simple-wave for a second time, it rendered everyone in Paris dumber than a bowl of dead fish. If the kids didn't interfere, Simpleman would have gotten their miraculous and Shadow Moth would have technically won. Thing is though, Shadow Moth was effected by Simpleman's powers the same as everyone else, so it's likely that he would not have been able to retrieve the miraculous, undo Simpleman's akumatization or even know what to do with the miraculous if he got them, rendering the whole thing pointless.
That would've been the case, but with Shadowmoth simplified as well, he himself didn't have much time to consider the fact he couldn't use any Miraculous properly anymore. Simpleman not knowing why he wants them aside from pretty jewelry (and likely not complicating himself knowing the answer), it wouldn't have come up until Gabriel got his hands on the Miraculous.
