I would gladly welcome CX into Star.

I don't know how many times I've seen Cathay flights that were priced right or had better schedules for me intra-Asia but I ended up taking something else like CA because Oneworld is far and wide my least favorite alliance. I've even taken CZ over CX because at least I can still see some value in Skyteam (benefits, not necessarily airline member quality).

As for SQ, it wouldn't hurt my feelings if they left. They don't play well with other Star members anyway. A large portion of their eco fares don't earn any miles and they give very scarce premium cabin availability to other *A programs for redemtion.