Prescriptivism is mostly just an unprincipled mishmash of shibboleths someone pulled out of their rear end hundreds of years ago, classism, and knee-jerk reactions against language change.

For example - why do people distinguish less vs fewer to refer to countable vs uncountable nouns? Because someone wrote in 1770 that they thought that distinction was elegant, despite not actually reflecting the way English at the time was spoken.

Why is ain't "not a word"? Because it originated in the speech of poor people, and was used less commonly by rich people. People roll their eyes at new business-speak because it comes from rich, powerful people, but look down their nose at language innovations from poor hillbillies and other disfavored groups.

And you can find writings from old prescriptivists complaining about literally every change in the language, such as hating the new ambigious use of singular 'you' when 'thou' was perfectly good and unambiguous or hating phrases like 'very pleased'.