So in other words you are saying that it is a lawful exercise of the president's power to commit felony obstruction of justice. You have not thought the consequences of this through. If he can commit felony obstruction of justice can he have the FBI and/or administration members go over and secretly intimidate witnesses not to testify? Can he go through and purge all the records of his criminal activity? If so, how would Congress or anyone else ever know that Trump and/or his associates were committing crimes or abusing power? Hell, what if Trump had his friend go over and literally murder every member of Congress and then told the FBI to drop the investigation? Can't impeach him if you're dead. (I know that's an extreme example but I see no limiting principle in your argument that would prevent it)
Congress simply does not have the technical capability or experience to properly investigate large swaths of federal crimes. Particularly relevant to this case is that Congress has no capacity to investigate money laundering, counterintelligence investigations for conspiracy with hostile foreign powers, or other financial crimes Trump, his family, or his associates have engaged in. When Congress investigates that stuff these days they do so by... you guessed it... asking the executive agencies like the FBI for help. If the FBI just said no and refused to investigate it Congress would be stuck either impeaching the President based on functionally no evidence or letting it go.
This is basically a recipe for unlimited criminal behavior by the president and his associates.