I think it was stupid of him not to hold a by-election for
Jurong GRC when Dr Ong Chit Chung passed on. It would been a good
gauge on the sentiments of the electorate, even if you lose
Jurong GRC, you can improve on your policies to win more votes for
the General Elections. You might lose a battle, but you would have
won the war. 
I don't think he dare risk putting up Ang Mo Kio GRC for
by-election because his seat is in Ang Mo Kio GRC, he risk not
getting elected. 
Seriously, if I were him, if it was any other GRC other than Ang
Mo Kio GRC, I would have risk it to get a better understanding of
the electorate. Unless of course they are 100% confident of winning
the next election, then the statistics gained from the by-election
would have been redundant. 
Don't you know that the PAP have always been very confident of
themself?