I don't quite remember anymore why Yang Kai was forced to break through onto the fifth rank, but I seem to recall he annihilated the clans that forced him to do it. I hope I remember correctly.
Dude, you have a rubber boot's memory. He did no such thing. Instead he's all happy in front of those sects, bowing down and praising them. The worst part of this story isn't the cave heavens and paradises forcing lone cultivators or minor sects to do that (even though it's incredibly selfish but also destructive considering the ink war), but the worst part is that the supposedly petty Yang Kai, who never forgives anyone who once slighted him (that's what he was infamous for, back in Star Boundary and before), simply forgave the cave heavens and paradises without a second thought. That broke his character miserably.
That being said, interestingly enough there's an explanation for this. Back when Yang Kai broke through to the fifth rank, that whole "you can only level up twice" wasn't touted in the manner it was later. Instead, it was mentioned multiple times that exceptional methods can bypass the limit. After all, Yang Kai immediately used one such method, the world fruit. But that wasn't the only one. There was a person who told him he can reach the ninth rank, though I didn't write down the chapter. It wasn't the only time. It's only later when the author decided to start to enforce the limit more strongly, to the point Yang Kai himself repeating you can only level up twice, despite at the same time knowing he can do it thrice already, up to rank eight, without needing to do anything about it. The author merely made him forget all the instances when he heard he can keep going. So, this whole thing is partially retconning. Logically those old monsters should know there are ways, but the retcon effect hits 'em hard.