Given the downsizing trend in the coming years,
I can see the A321/A321neo have a place in Cathay's fleet.
Older A320/A321 are going to retire, perhaps some will get transfer over to UO
There has been a lot of overlapping between CX and KA anyway.
They used to be two independent company and are competitors, mostly on Mainland China and SEA routes.
Since CX had acquired KA, KA has become CX's regional division focusing on mainland china and supplementing secondary regional routes which are too thin to fill CX's smallest plane (A330).
Although UO target the leisure market, it provides some extra capacity on thinner routes, filling some secondary markets. KA kind of sandwich between the two... I wouldn't say it's redundant,
CX kept the "Dragonair" branding because of its provenance in the chinese market, but after all these year operating as "Cathay Dragon", they have finally decided to retire the dragon from its "shark fin".
I don't see any reasons why Cathay cannot keep the A321 around flying under its banner, there are still routes that are too thin to fill an A330, especially given the current climate.