Since TT is already a very rich man, I wonder too why he wants to be president, surely its not about the money. My reasoning is he is there to cover the PAP's mistake. Thanks to the internet community we all know of TTs dirty linen which has been exposed to the public. For example he favours foreigners to singaporeans etc. He is not fit to be the President. But he has place himself in the running to win. That way he and the PAP continue to dominate the country. Also if the Presidency falls to TJS or TKL etc, TT's dirty secrets on his role in the PAP and all the other companies will be exposed even more. That is why to the PAP and for TT's own future, he must win.
Also another point, Nathan is highly unpopular. TT would have won if he chose to stand at the last Presidential election, but using him now to stand for Presidency has just gotten so much harder because of the online community. Also TT and son's explanation of what is going on is weak and does not hold water. When TT becomes President, unless he has a change of heart and fight for Singaporeans which I doubt will happen, he will be even more hated than Nathan. History will not look on him favourably.
TT to save himself further embarrassment should walk out of the presidency, say he is too ill, family business needs him etc and at least the hounding will stop as we will have nothing else to say. If he wins the presidency, the presidency and him will be even more tainted than it is. If he looses the election, the PAP and him will really have no face.
Both options is bad for him, if he wins, he looses. If he looses, he really looses. TT just walk away, you will have preserve the limited dignity you have left. and the hounding will stop.