I have to disagree with this. The Skip Barber series may be a "D" series, but some of the best drivers in iRacing can be found there. The series is so big and competitive that you have to develop some decent skill in order to stick around in this car.
The GT3 challenge, however, is a B-level series and is an absolute wreck-fest at all levels. The big grids, variance in skill level, and sheer speed of the car lead to a dozen retirements per race. That's not an exaggeration. Anyone who's raced in GT3 has seen his F3 screen fill up with blue names when crossing the start/finish line, a list of all the people who wrecked (or got wrecked) in one lap of racing and are now getting repairs.
We need to get away from this idea that your license is somehow indicative of your skill level. You can fast-track to an A-license by doing nothing but time trials. You can literally do it in a week if you're dedicated enough, which means you can earn iRacing's top amateur license without ever driving against another human being on track.
A much better indicator of your skill is your iRating. If promotions were based on iRating instead of safety rating, the only people who earned A-licenses would be the guys who belonged at that level. That would never, ever happen (too many people would leave the service when they realized they'd never be able to compete in the higher classes), which is why bad driving will continue to exist at all levels of iRacing.