Quick Question: Why did Joe Biden fire only 1 corrupt Ukrainian official?

Democrats and the #FakeNews media are saying that Biden isn't corrupt because the prosecutor he fired was corrupt and "everyone" wanted him fired.

They say that the fact that that firing ended the investigation into Burisma, the company that paid Hunter Biden millions of dollars, was just a coincidence

The question that that position raises is given that at the time the US had identified many people in the Ukraine government as being corrupt why did Biden use a quid pro quo to only fire the one corrupt Ukrainian who could hurt Biden personally?

Why didn't Joe use the $1B in aid to get other corrupt Ukrainian actors fired too?

And if Joe was really waging a relentless war against Ukrainian corruption rather than paying back Burisma, the corrupt company that was being investigated and which had paid Hunter Biden millions of dollars, why didn't he demand that the investigation into Burisma be continued by the new "non-corrupt" prosecutor?

Which leads us to another thought.  If the new prosecutor wasn't corrupt, and if he was corrupt it renders Biden's actions useless, why did he drop the investigation into Burisma a company that everyone then and now says engages in corrupt business practices?