I know that its just an in-game mechanic, but from an in universe standpoint, releasing trained pokemon into the wild is a terrible thing to do. The pokemon would either wreck the local ecosystem or perish in an actual survival situation. Say that you caught a purrloin on route 1, raised it to a high level, realized what a shitmon liepard is, and released it. While liepard may be a shitmon in league sanctioned combat against mons of similar power, its not fighting opponents of similar power. Its hanging out on some low level route, destroying local trainers, hunting local pokemon, and defeating any natural predators it may have with coverage moves.