Speaking as someone coming from an unrelated field, I can assure you that being uber bright and being good with people (both skills REQUIRED for success in medicine) gets you most of the way toward the skillset necessary for any other profession. I don't see drastic differences in ability between my med school classmates and the folks who excelled in law -- actually most of my med school peers are better in a lot of aspects. Most of the "skills" people perceive as necessary for being a lawyer or banker are learnable, not inherent, and nobody is a better learner than a med student. Folks are too willing to sell themselves short on SDN by comparing themselves to average folks. You have to look at the person who had as one of his choices attending med school, and compare to him. That is a very small select group, and generally successful.
I think the flaw you and everybody keep hitting is the "guaranteed" language. The first thing you get taught in any finance course is that there is generally a risk reward ratio, and the more unlimited the reward you seek, the greater the risk you often have to be willing to take. Things are valued based on this -- people will be willing to invest in greater risks if the reward is great enough. So too with employment. If you are willing to step away from the guarantee of a particular salary range, and take a career risk, the ceiling is often higher. Which is why folks go into eg investment banking. There is a lot of washout in the field, and plenty of people don't make it rich. But the sky is the limit in what you can earn if you are successful. Not so in things like medicine which has a rather finite salary, based on the number of hours and how much you can get from reimbursements. The second thing you get taught in any finance course is the time value of money. A dollar today is worth a LOT more than a dollar a decade from now. So if you are going into a program that takes a decade before you start earning, and even worse, paying out six digits for the privilege, you often start out in a hole that is impossible to dig back out of. You will be old and gray before you catch up to a lot of your biglaw and I bank and dental peers, if ever. Will you overtake them at the end? Perhaps, but that depends a lot on your field of specialty, something you can't always bank on. (best to assume you will end up doing primary care with an average current salary of $140k because a very substantial percentage of folks who go to med school will end up in this range).
And the final straw, not something you learn in school but something you need to be cognizant of, is that as reimbursements are slashed in medicine, and tuition is going up, you are seeing folks in medicine ending up substantially worse off than the prior generation of physicians. This isn't your father's oldsmobile. And lots of pundits predict salaries to continue to creep downward. At the same time as medicines salaries have been declining other professions have seen about an 8% increase. The various news articles posted on SDN in the past year highlighting FPs who have to take on second jobs and sell things on E-bay should be read with some concern. Plan to be comfortable but not rich in medicine. There are more folks who end up in the "rich" category in other fields. But no, it's not guaranteed. Nor should it be -- it is a question of how much risk you are willing to take for the reward. But if you are going into something for money reasons, you should be far less risk averse.