What was the point of having Mia Goth play the old lady?
Normally, when you have a young actor playing an old character, there's a specific reason for it, such as the actor playing the same character at different ages, like various actors in Back to the Future Part II did for example. Otherwise, you'd normally just hire an old actor to play an old character.
In the case of this movie, we get the undesirable young-actor-in-fake-looking-old-age-makeup-with-a-fake-sounding-old-age-voice effect, in exchange for what?