And yes, the CCP is effectively unoustable but the CCP is the government, not just a Western style party, it's a complex governing body. For factions and individuals within the party there absolutely is an incentive to perform, and the party had countless of purges due to unrest.
Don't forget that Xi himself was sent to the countryside under the program of the same name after his father was thrown into prison. The party isn't going to go away, but the authorities in it absolutely are subject to scrutiny. Also Taiwan of course was subject to one party rule until the 90s and not a democratic nation either, and neither was South Korea under Park Chung Hee. Economic development in all three countries did not take place under democratic government.
It is a "justification" but since failing to do it does not come with any penalties there is no actual accountability. Mao starved 50 million of his comrades by mismanaging society and outright killed a few million more being paranoid -- and they didn't kick him and his cronies to the curb, instead they venerate him like a god and give his friends' grandchildren sinecures.
The fact that China lagging decades behind South Korea or Taiwan for 50 years didn't get the CCP ousted tells you how thin the veneer of "accountability for rising living standards" is, and how much the party actually rules with an iron fist.