Xi did wipe out all possible challengers. It is convenient that they were indeed corrupt. All in Politburo are actually corrupt, or abusing their power. If somebody overthrows Xi, they could also find enough to hang him multiple times. I think he is set for life, unless he is forcibly deposed. His plan to modernize PLA is also a move to cut down ground force which can be a threat to his power. You need boots to stage a coup and consolidate power.
There’s really no such thing as an incorrupt communist, since communism is corruption.