This is not an argument that whatever the President believes is in the national interest should be allowed. For example, this specific argument of Dershowitz is not saying that if a President thinks that dissolving Congress is in the national interest and his personal interests, it's okay to dissolve Congress. It's a different point.

Language is a funny thing. No matter how an author tries to make something clear and precise, people will always quibble about the meaning. I interpret Dershowitz's claim as "it is not a crime for the POTUS to do anything, that is in the 'National interest', as he sees it". So, it would not be a crime for Trump to pull a Charles I and dissolve Congress, or, declare himself "President For Life". What matters, is who has the stones to stand up to him, and say "NO", and the power to make it stick. We old phartz remember the "Saturday Night Massacre". Nixon ordered AG Elliot Richardson to fire Archie Cox. Richardson resigned instead. Nixon ordered Assistant AG Bill Ruckelshaus to fire Cox. Ruckelshaus refused, and resigned. Nixon ordered Bork to fire Cox, and Bork did as he was bidden. Neither Richardson, nor Ruckelshaus, had the juice to make their refusal stick. Just like Bill Barr did not have the juice to demand Trump give up on overturning the 2020 election, so he resigned. By 28, Trump will have had 8 years to ponder what he would do with/to state officials, like the GA Secretary of State, if they do not do as he orders. Trump something up on them, bankrupt them by hauling them into court until they are overwhelmed by legal bills?

Steve