When you keep in mind how Iris and Colonel where meant to be a "super reploid" but got divided in two with no other upgrade to fill the void.... both of them come with very critical problems that ended up sealing their fates
Colonel took all the fighting power, but lacks Iris open mind. This means that once the guy set on one thing, no one can tell him to back off (this results in a blind pride that ended with the battle vs Zero even if there was other ways to solve everything)
Iris on the other hand is the "heart" and takes everything for the best possible outcome, but lacks any sense of a fighter reploid. This means Iris is not able to understand the meaning behind Colonel and Zero fight and took this as Zero betrayal for killing her brother (and even in the end, she just decided to go for it until her death)
The only reason both are popular is that Colonel looks cool, Iris.... because she is Zero ship for the fans to meme
If said super reploid actually worked, then Sigma most likely needed to work harder to make the repliforce go Maverick instead of the joke of a reason that both pull in the end
But then again, we are talking about the same writers that added Iris as Zero assist when the repliforce was called Mavericks..... even if Iris IS a repliforce member and should be out of the hunters way