Dentistry is not on the decline perse, but it isn't quite what it used to be 10+ years ago. Incomes are on a steady decline as insurances slash reimbursements. Delta, UC, and UHC just shaved 15-25% of already meager payouts. We're at pre-1990 payment levels in my area. Most patients are brainwashed into needing to use their insurance and believing that is the only way they are able to get dental care. So there is definitely an income cap. As an associate, there is little job security unless you find that unicorn practice that needs someone and is willing to work with you. And the ROI is rapidly dwindling as student loans are ballooning. But the flexibility can't really be matched with working as much as you want to.
Medicine has more job security and higher payscales. But the hours are intense, the insurance hoops you need to jump through are crazy and the amount of training needed to get there are trying. I had a medical resident in my chair yesterday and I was surprised by the hours that she was clocking during her residency. And any speciality that you think you may want now is not set in stone. Lots of things are dependent on your grades, Step exams, and honestly, how you feel as you progress through your career in medicine. Of the people I know who went to med school, only 1 ended up in the speciality he wanted BEFORE he started school.
As far as corporate dentistry taking over the Midwest, they are popping up everywhere. Even in areas where you wouldn't think there was much need for it, they are popping up like crazy.
Both have risks and rewards. Just remember that wherever you end up, the grass is always greener on the other side. I'm happy with dentistry overall, but am hating watching corporate dentistry and the insurance companies run our profession into the ground.