This theory that a POTUS can't ever obstruct justice for any actions he takes in the context of his executive power - which has been endorsed by Dershowitz and Gingrich, and now many conservatives, has staggering implications. The POTUS could order an executive branch employee to do literally anything illegal, including to destroy evidence of criminal conduct on the part of the POTUS himself. He could order any and all investigations into his own conduct be shut down. He could give literally any order in violation of any statute, so long as the order didn't directly conflict with the Constitution.
According to this theory, impeachment is literally the only check on this otherwise unlimited power to commit heinous felonies with absolute impunity. Unfortunately, Congress doesn't have the resources or knowhow to the be the sole investigatory body. They do not have the resources of the FBI. So their ability to even uncover evidence of criminality is limited. That is not even to mention the partisanship issue - if the POTUS' own party controls Congress they may well be reluctant to hold him to account for anything and impeachment is, after all, essentially a political process.
This theory is extreme and totally unhinged. I don't think it's the slightest bit exaggerated to say that the fate of the republic is in doubt if this theory is ever endorsed by the courts. It's a license to commit unlimited felonies. Is this really what the founding fathers intended when they decided it was best to move away from absolute monarchy? I'm sure conservatives will suddenly change their minds about this the next time a democrat is in the oval, but this illustrates just how far the right is willing to go to exculpate Trump from any and all wrongdoing.