It isn't a feature, at least not for users. I strongly suspect youtube's autotranslate has more to do with regulatory compliance and content moderation. Rather than having people who speak X/Y/Z languages, they want each every video to be translated into English by default so they can be feed more easily into the the system that vets content. Having translated and non-translated copies floating around is probably seen as a needless complication.

Once a monopoly has been established, the next step is to actively make the product worse in order to either reduce costs or push users towards premium features.