147,
Rashi often says “makshin haolam”. We know what “haolam” means as it clearly does not include amei horatzim, rather it referes to the learning world.
With that established, it is clear that yiddish is neither archaic nor outdated.
Torah, Nach and seforim kedoshim are most certainly NOT written in ivrit. No one claims (other than you apparently) that ivrit is Loshon hakodesh. They are vastly different.