This paragraph reads as Trump asking Zelensky to provide information related to the origins of Russia/Mueller, in the context of AG Barr's ongoing probe. So at least here, he's asking for assistance related to a US investigation -- not a Ukrainian investigation 
If you believe an investigation into the origins of Russia/Mueller is legitimate (which it is, there are still plenty of outstanding issues) then asking for a "favor" in the context of that specific investigation is not unto itself untoward
Zelensky then brings up Giuliani, and Trump does one of his typical semi-intelligible riffs about how Biden maybe did something horrible, and Zelensky should talk to Giuliani. Bad form to even bring up Biden? Sure, but it's a lot vaguer than initially presented
The political question is, what's the best way to reproach a president who routinely exhibits "bad form"? How about an election, rather than an extraordinary Constitutional remedy that will consume the country for months and months?
That the first subject discussed is Mueller makes all the difference because it confirms there is direct narrative continuity between the Russia investigation and the Ukraine episode. So the whole Mueller/Russia saga is alive and well, and provides the narrative underpinnings
Any forthcoming articles of impeachment would have to encompass the Russia/Mueller investigation -- this is just the latest iteration of that story. Ukraine phone call cannot be separated from the broader Russia narrative that has dominated US politics for 3 years
