When Hu is retired, he needs gurantee that Xi is not moving against him or his political allies. Although that is the case 99.99% but Hu wants 100% gurantee. And this case is that remaining 0.01%. To a large extend, Hu is safe but you can not be 100% sure that his own side kicks won't be purged by Xi one way or the other.
Lai's case can be nothing or can be managed to shake all of Xi's power base. It all depends on the political situation. A tiny intern almost brought down the most powerful man on earth, imagine what a wide spread and connected corruption case could do. Don't forget: this case can be painted as corruption case but it can also be painted as traitors and military security violations because trafficking was invovled with Taiwan during the time of hostility between mainland and Taiwan.
It is deep water. :)
