Modern batteries have much longer life than the phone itself. Think about it, when did you last hear someone had to change their phone's battery because it ran out?
Typical phone lasts 2.5-3.5 years, after that user buy new one, and by then phone battery's life is around 70-80%.
Besides, there's no such thing as overcharging. There's an internal circuit in all phones/laptops which cut off power to the battery once it is fully charged. You can easily test this by touching the phone when it's charging. During charging it will be warm, but 10-15 minutes after it's fully charged, it will be cooler, indicating it is no longer drawing power from the charge
Yes, there is no overcharging. And Li-Ion and Li-Po batteries dont run out because of that. Lets assume a case, a phone has charged to 100%. It will stop charging the battery now and run on battery power. As soon as battery drops to 99%, it will start charging it again. this cycle will continue. As there will be no complete Discharge-Recharge cycle, the battery will start loosing its life.
Best way is to charge till 80%, and once its 30%, set it to recharge. And Do a complete Recharge-Discharge cycle atleast once a month. I.E. discharge to 0 and charge to 100.
But if someone wants to use phone for more than 3yrs, he will have to get the battery changed. But heres a contradiction, as battery capacity is increasing, i.e nowadays every phone has a minimum of 4000MaH battery, that even after loosing about 30% of charge, will not majorly affect it. but then there a multiple environmental factors in it.