The ring times are set by the carrier, hence the need for the dialer codes to change it. iPhone users have the same amount of time to answer incoming calls as Android users (by default). I don't see it as a money grab, not everyone wants their family or friends waiting forever for them to answer the phone if they don't hear it within 10 seconds.
If anything, this "slider" you suggest is something the carriers could implement on their website, or as part of the dial-in voicemail system.