Of course it is not that it’s somehow a “stand in for he or she” inherently in current usage. It’s just that it has recently replaced those other pronouns in places where for some time they had held near-universal prevalence among most users of this language.

Just as some people who’ve never known the old ways think those people who still aren’t accustomed to it are putting on an act when they say it’s weird and confusing, I suppose it would be easy for those who’ve lived through the change to mistakenly assume that young people are being disingenuous when they act as if there’s been no change for hundreds of years and there’s nothing to remark on here. If you’re old enough to have seen it happen, the change in usage seems very obvious. If not, perhaps it isn’t.