The Supreme Court in Nixon vs US did not rule that the exercise of President powers, such as firing Executive branch employees, could constitute a crime. It ruled that the general claim of Executive privilege for all documents and records was not absolute, but could be set aside for specific items subpoenaed in a criminal proceeding.
Trump voluntarily had thousands of documents turned over to the FBI and the Special Prosecutor. He even let his counsel testify for hours. He went far beyond what was necessary.
Further even the articles of impeachment against Nixon did not mention his firing of Executive branch employees during the Saturday Night Massacre. The broad bipartisan opinion then was exactly what Barr and a host of constitutional scholars claim: that the President cannot be impeached for lawful exercise of specific Presidential powers.
Note the Article 2 offenses involve allegations of Nixon ordering Federal agencies to take illegal actions.
Your attempt to paint Barr’s explanation of obstruction as a reversal of Watergate-era limits on Presidential powers is just incorrect. His interpretation is quite consistent with the prevailing opinion back then. If anything, Trump’s voluntary cooperation with document and executive branch witness testimony requests, completely undercuts any predicate for alleging obstruction based on violation of Nixon vs US.