Seems like people confuse or something about their 'regional subsidiary' and 'low-cost subsidiary'. SIA and Scoot is definitely not a relation of Thai and Thai Smile. Scoot is just a low-cost airline of Singapore Airlines Group, and was not established just to serve routes that SIA don't serve (although they have their own routes but they share some destinations). CPA and HK Express is a bit different (since they acquired them from HNA Group) but not a big difference either.
And even if they exist to fly to the destinations out of the main carriers' coverage, it's still better than not having direct flights at all. I mean, Silkair and Cathay Dragon were still a full-service carrier, so the fact that SIA and CPA kept most of their flights mean that they're profitable and competitive enough to be served by the FSCs. Scoot and Cathay Dragon are LCCs, so nowadays, especially in Asia, are more sensitive and competitive in terms of price. Let' say, hypothetically, people flying btwn. Busan, South Korea to Hong Kong, a route which HK Express exclusively serve (no Korean carriers serve this route as of now), for travelling wouldn't be so happy to be forced to fly more pricey CPA or transfer at somewhere else. Scoot and HK Express have some routes that they exclusively serve, and there's reason why SIA and CPA don't serve that routes.
In my opinion, both SIA and CPA should keep Scoot and HK Express, as they offer cheaper flights, and it's still better than not having direct flights at all. But partnerships with SA carriers, just like what Eurowings does would be essential...