Are you some kind of clown? U are hilarious.
The Presidency even before the advent of the Elected President has been more then a custodial role. The Presidency is endowed with certain powers, which the PM or Parliament do not have. ANy blind fuck can see that. For example, the President is the only one who can sign bills into law, pardon prisoners, and since EP, allegedly oversee the national reserves. For example, if the President decides the budget send to him for signing by the govt includes too much money for their salaries, he can refuse to sign it into law. And send it back to the govt to be re-worked. You don't think this is power?
The Prime Minister does not derive his legitimacy from the Parliament. That is not how the Westminster system works. The Prime Minister is part of the govt. which is separate from the legislature (which is the Parliament), and both are separate from the judiciary. These 3 branches in a real Westminster system acts as a check and balance for each other. The Prime Minister is a person nominated by his party to lead the govt if the party wins the election. In many democratic countries, the party forming the govt do not even have enough votes to pass what it wants through Parliament. So, the PM is hardly a source of power or legitimacy from the Parliament. Many PMs are removed from power if their party forms a coalition with other parties in the Parliament and the coalition fails.
And finally, Goh is not suggesting an evisceration of the Constitution. If you know anything about the Constitution, you would know the PAP has done a fine job eviscerating it through the decades, The original version of the constitution (which I studied in the early 80s) bear no resemblance to the one today.
And I have a hint for you. If the EP is such a custodial and ceremonial position, why would the PAP go through all the lengths (e.g. ridiculous criteria, vetted by PAP controlled PSC, racial rotation, etc) to make it difficult and hard for a non establishment person to qualify to run in it? The answer is that surprise, there is actually power in the EP.