The Constitution provides for indictment/impeachment to be exercised by the House of Representatives and trial/conviction/removal/acquittal be exercised by the Senate. There is NO other provision, other than the ballot box, to discipline a President for wrong doing while in office. And that is how it should be.
If SCOTUS does not make that ruling, then all Presidents now and in the future will be/could be subject to endless vindictive/malicious investigation, legal persecution, prosecution and be tied up on court forever. Not only will he be in fear of legal repercussions from every unpopular decision--and pretty much everything is unpopular with somebody--but he will be physically prohibited from doing his job.
SCOTUS should rule immunity from prosecution for the President in the course of his duties now and forever other than what Congress chooses to do.