BA/BS->MD/DO programs do require students to earn a high college GPA and high MCAT score to retain the assured admission to the MD/DO program, so the student cannot just be a “regular college student”. If a college does what you are suggesting, it will probably impose a GPA, course selection, and test score minimum to maintain the assured admission.
However, that presumes that the most impressive 9th graders will be the most impressive 12th graders and college students. Considering that 9th graders are even less independent of parents than 12th graders, such a program would probably select even more for parental involvement and knowledge than current college admissions processes. I.e. the credentials of a 9th grader are more dependent on parental circumstance and support than the credentials of a 12th grader, and even the act of applying to such a program as a 9th grader is even more of a parental decision than the act of applying to a college as a 12th grader.
Also, the college would have to find some way of assuring financial aid predictability for the parents early on. (Medical school is different, since students can and are assumed to have to take $300,000+ loans for medical school.)