Are you sure you're wrong?

The relevant sentence seems to be "If a participating developer surpasses the $1 million threshold, the standard commission rate will apply for the remainder of the year." (emphasis mine)

Apple takes the comission as you go, so it sounds like they take 15% of each sale until you hit $1M, and then 30% for every sale after that.

The next year they'd take 30% right from the start though, so a good year followed by a bad one would be unfortunate.

Except then the next year you get kicked out of the program and back to 30% on every dollar?

No, you start by 15% default until you reach the $1m threshold. After that you pay 30% for the remainder of that year. Please correct me if I'm wrong.

That's only if you're in the program.

If you get kicked out, you're stuck paying 30% for at least a year.

That is correct, but then the next year you start at 30%. So if you make slightly less next year you will end up earning less money that year since it is all at the 30% cut (And then the following year will fall back down to 15%)