CCS is the familee's designated seat warmer, not HSK. But CCS screwed things up so badly for himself that it would have caused internal strife within PAP if he was DPM.
So HSK became a convenient choice while CCS repairs his image (but ended up making more gaffes).
How to then clear the way for CCS again? Losing an election would be a good reason.
Hence HSK is the sacrificial lamb thrown into East Coast.
If HSK wins, well and good for the familee as the PAP does not lose East Coast GRC. HSK can always be "retired " from minister anytime because of his stroke condition. He will never be an obstacle to CCS becoming PM eventually.