A vote by 51 Senators to dismiss an impeachment should be a valid action in light of the fact that the Constitution requires 2/3 to remove from office.
I don't disagree.
However, the devil is in the details.
It does appear as if the Founders intended more than just a debate and vote by the Senate. Having the Chief Justice preside must have a purpose. The Founders expected a trial. From the Constitution, "The Trial of all Crimes, except in Cases of Impeachment, ...".
The implication of that quote would seem to be that an impeachment MUST be a criminal trial. That has incredible consequences for Trump. He is to be afforded every right that any defendant has (except the Senate is the jury). That would include a presumption of innocence and the right not to be a witness against himself. Does Executive Privilege apply to such an impeachment trial? Hmmm...
I think this creates a strong presumption that the presiding judge, the Chief Justice, CAN dismiss for lack of a specific crime. "Gross Unpopularity" won't cut it.
I think it unlikely that Chief Justice Roberts will create a new exception to the inadmissibility of hearsay. I think it unlikely that the Chief Justice will allow a witness to "presume" something which he didn't witness. I think it unlikely that he would accept a charge of "Contempt of Congress" for Trump suggesting that the Supreme Court should rule on the applicability of Congressional subpoenas. Otherwise we would have to believe that the power to issue subpoenas by Congress is unlimited.