I have a friend who works at a battery/electronics chain, and replacing phone batteries (and screens) has become a substantial part of their business. I couldn't say what percentage of the population knows it's possible but, when your phone breaks or the battery dies, I'd bet a fair number of people google repair options, and from there going to third parties seems fairly common.
Almost everybody using one? At least in Europe are third party stores that replace iPhone batteries (and repair other such damages) all around the towns.
It should be noted that Apple has always been very against third-party stores, and have tried to punish consumers that use third-party stores in an attempt to discourage people from using third-party stores. Not to mention that Apple intentionally makes it hard for third-party stores (not selling parts or even providing information about parts).
