Up until recently, there always seemed to be a marked difference between the way people spoke vs how they conveyed thoughts in writing. These days, it often feels like most writing is just conversational and stream of consciousness and differs little from how many people speak.
It always makes me curious how we generally view the people of antiquity as speaking very eloquently and properly, but that's probably because we only have writings from their time, not recordings of how they actually spoke.
tbf with the advent of text messaging and the internet a much greater proportion of text is conversation
I'm surprised you mention us not having recordings of the older times being why we thought people spoke eloquently. Even in old recorded audio and video, like old TV, it feels to me like people speak much more eloquently.