It’s not just DL and SkyTeam; Alliances in general have come to matter much less in favor of JVs. And why not- the airlines are simply directing traffic to where the $$$ is for them.
I agree that JVs are where the money is at. But to the extent that alliances are all about extending your network, it seems very antagonistic to block any TPE connections on CI and not sell codeshare connections out of SEA.
Perhaps it's because of the power of the SFO hub or facing stronger Asian carriers, but UA will still sell connections w/ SQ and BR and offer Polaris lounge access, despite the direct competition.