You've managed to list exactly the reasons why a beginner should
not have a 600cc sportbike (a very unforgiving machine), but have somehow also managed to convince yourself that those reasons don't actually apply to people when they're outside of the heavily-controlled class conditions. The inherent unpredictability of real-world riding makes it
more important that riders stick to 500cc or less if they are getting sportbikes when compared to the MSF class conditions.
As has already been pointed out, the class is not a substitute for experience. You have apparently missed
this link in its entirety. I have never met a rider who didn't make a mistake every now and then, especially during the first three years to five years of riding.
A 600cc sportbike isn't going to tolerate mistakes well at all. Hit a pothole and accidentally twist the throttle? Been nice knowing you. Panic and grab too much front brake? I wish you luck my friend. Encounter tramlining for the first time at 70 mph on the freeway with a sportbike's inherent twitchiness? Here's hoping that you recover.
You might as well suggest that a person who did well on his driving test just go right out and get a 427 Cobra for their first car. I don't care how well a person has done on basic driving or riding tests or classes, a racing machine that is only just barely made street legal is a bad idea for a first vehicle. It doesn't matter how much a person might think he's better than most.
An interesting statistic is that 90% of drivers believe they have "above average" driving ability while also believing that 60% of the other drivers have less skill. Notice how those two percentages don't even come close to adding up. Almost everyone overestimates their own abilities.
And, as I said, if you ask the people who are actually running the MSF course, they will tell you that buying a 600cc sportbike for a first bike is a bad idea.