Noni Madueke's arrival wasn't to replace Saka, but rather as a backup player to ease Saka's burden and fill in for him when he's injured.
In my opinion, Noni Madueke hasn't impressed much so far, and he hasn't fully established himself in the Premier League. However, he likely has the potential to develop in the long term, and we'll see whether Mikel Arteta makes the right decision, considering this is a controversial decision and one that many Arsenal fans disagree with.
You are right that Madueke's arrival at Arsenal was not to replace Saka; the player is not at the level to bench Saka in an important match, but Arteta recruited the player as a backup for Saka in a situation where the player is not in good shape to play. We saw how much Saka absence impacted their performance previous season, since they were unable to defend any titles. I said before the start of last season that Arsenal were in need of a left winger to replace Saka so that he could play at his best for the rest of the season.
However, Saka absence has had an impact on their games, even though they have Ethan Nwaner, Arsenal fans are upset that the team bought Madueke since he has never played at that level for the money they paid. But if Saka is unable to play, can Madueke bench Ethan Nwaner? I'm just curious, and I hope Arsenal wins a title next season with the players they're gathering.