It is in the US interests to investigate the Biden family corruption and that of other Democrats who have used family connections to profit while evading scrutiny

Adam Mill:

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... Is it a matter of legitimate public interest whether Vice President Joe Biden sought the firing of the Ukrainian prosecutor in order to benefit the company for which his son worked? Did Biden engage in a “quid pro quo,” i.e., protecting Burisma in exchange for his son’s outsized salary for board membership? That seems like a question of vital public interest. And the interest of the public in that question shouldn’t change depending on the political allegiance of the people involved.

Does the fact that Biden is also a potential political opponent of Donald Trump confer some kind of immunity from scrutiny? It certainly didn’t when the Obama Administration used the same January 2016 meeting to pressure Ukraine to end an investigation into the Bidens and launch an investigation into Trump’s campaign chairman, Paul Manafort.

Also, as noted by John Solomon, Ukraine produced a forged ledger that falsified evidence of corrupt actions by Manafort. The story of the ledger was then leaked and reported by the press in a successful effort to embarrass candidate Trump and drive a wedge between him and Manafort. The forgery thus had a tangible impact on the 2016 U.S. presidential election. Ultimately, the ledger was so obviously faked that Special Counsel Robert Mueller’s team discarded it during Manafort’s trial.

Attorney General Bill Barr has launched an investigation into the Russia collusion hoax and the phony Manfort ledger is a relevant topic. Barr did not approach Ukraine directly but instead, per protocol, requested an introduction and sponsorship by the president of the United States to all foreign leaders from whom Barr might want help. The phone call between Trump and the president of Ukraine was simply that—a heads-up to the Ukrainian president of the investigation and a request to cooperate.

The true purpose of the impeachment inquiry is to protect the business model of turning power and influence into cash. If the Bidens go down, who knows who might be next? The Democrats are attempting to use the “thing of value” theory of election law to stymie a legitimate inquiry into political corruption. Because asking questions might hurt Biden’s chances in the 2020 election, Trump can’t ask them. Or so the argument goes.
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The impeachment inquiry is looking like a coverup of corruption based on the Democrat model of using their children as proxies for foreign payments.  Biden is not the only one who has done this.  Pelosi's son also had a deal with a Ukraine company.