That's not exactly what happened, people in Ukraine were against this prosecutor and protested for his firing because he was obstructing actual cases against the former government. Biden also wasn't the only person calling for him to be fired, other western countries also thought that Ukraine's anti-corruption efforts weren't serious enough.
https://www.nytimes.com/2016/03/30/...-prosecutor.html?auth=login-email&login=email
So your characterization of the "fire the prosecutor that's investigating my son, or you don't get your $1 billion." is wrong. Also his son only joined the board of the company being investigated after the investigation had began.
In the firing/resignation of the prosecutor you've got several western countries/insitution calling for his resignation and in Trump's case you've got a man putting pressure for an investigation into a political adversiary that doesn't seem to be founded on anything and seemingly only benefits him.
It's funny how the republican party which use to be about morality has gone straight to the "but was it against the law?" defense now.