Used to replace their lack of vocal range? Really? I guess you were there when they were recording their respective parts.
Actually, this song is easily within both of their ranges. Plus, that song is not hard to sing to begin with. The use of Autotune here was not done to fix an off key vocal. No one really uses it for that anymore, anyway. It's used to give the vocal in question that "sound" which is popular in electronic styles of music these days. Just like a ton of reverb and triple stacking each voice on the hook was popular on R&B vocals in the 90's, so is this effect hot today. It's a fad.
This is a misconception from people who don't know modern vocal production/mixing techniques. They think if they hear Autotune on a vocal it automatically means that the singer can't sing and he or she is trying to cover it up by using the program. Most of the time, especially in the last few years, that's just not true. If they were really trying to fix out of key vocals they would use a VST called Melodyne and you'd never know the difference. It sounds totally natural when used by someone who knows what they are doing. Totally transparent.
Besides, you really can't compare what Chris Brown and Arianna Grande are doing to Luthor or Anita Baker. Totally different styles of music appealing to different audiences.