I can't see him not winning another major to be honest, too much talent in his long game. I also can't see him winning more than two more.
Golf is currently being played at a higher average level than any other era, throw prime Tiger in and I reckon you would cut his major tally by a third, Jack by more than half. So essentially dips in form are far harsher on results, Spieth is coming out of a horrendous dip and he's not the player Rory is and lesser players with better consistency like Rahm, Schauffele and Reed are always in and around the top 10. With at least 6-9 (yup) prime years ahead of him he'll have one or two more dominant seasons, majors could miss him sure but he's overdue.