Digi-Key has a good article explaining LED lifetime here, with graphs showing how operating conditions affect lifetime; for example, they show a 1% decrease in brightness over 7,000 hours at 25°C, but a much larger 11.5% decrease at 85°C, a typical operating temperature in an LED replacement bulb; the LEDs used in cell phone backlights are just your standard low power type LEDs (e.g. 20 mA, 50-100 mW power). Even for high-power LEDs, I suspect that the vast majority of lamp failures will be due to the ballast electronics, plus even if the LEDs in a cell phone lasted for only 50,000 hours, that would vastly exceed the lifetime of the phone itself.
Also, OLEDs are based on a different technology (organic polymer, basically plastic) than traditional semiconductor-based LEDs (which are also used in displays, as in large electronic billboards, but presumably too expensive or difficult to use in miniaturized displays).
Of course you’re not cramming 120v into the phone–that’s not what I’m saying. My point is just that in theory there’s enough power from the outlet to both charge the phone at maximum rate and light the display at the same time, if the phone is designed to take advantage of a charger with more than one watt. Granted, it’s not likely to be doing that, but it’s possible.