As far as I understand, keeping a mobile phone plugged in would draw current from the source (i.e. an USB port) rather than from the battery. Therefore, the battery would not be used as long as the phone was plugged in, meaning that it would not experience any current draw. Given that batteries tend to "die" faster when they are used too much, this would mean that if a phone was plugged in most of the time, the battery would last longer because it would take more time to experience an amount of charge-discharge cycles such that its capacity starts to decrease.
Is this idea correct? If so, is there any evidence that supports this? By evidence I mean some kind of circuit or any other physical explanation that is not just based on common sense or intuition.