The answer is simple. Section 65 of the Constitution does not provide for election if the president were to resign. If there is to be election within 90 days the Constitution would have to be amended. It is important for the readers to know the content of Section 65 and then explain why it cannot be amended without the consent of the president unless we want a referendum.
The relevant portion reads:
"(2) Whenever the office of President becomes vacant in the circumstance set out in subsection (1), the Vice-President, or if there is no Vice-President in office at the time, the Speaker shall assume the office of President for the residue of the term of the former President."
Why then is it impossible for National Assembly members to amend the Constitution unless it risks the holding of a referendum?
The answer is simple. Section 226 Subsection (3) reads:
"If the President fails to assent within thirty days to a Bill passed by the National Assembly in accordance with subsection (2), the Bill shall be returned to the Speaker who shall refer it to the Independent Electoral Commission. The Independent Electoral Commission shall cause a referendum to be held on the Bill in accordance with subsection (4) and, if the Bill is supported on such a referendum by the majority of voters provided for in that subsection, it shall again be presented to the President for his or her assent."
Hence a bill passed by the National Assembly on constitutional amendment may not be assented to by the President to become law. In that case there must be a referendum before the president could be compelled to assent to the bill.
Those who blame National Assembly members for not amending certain constitutional provisions are simply disregarding or are ignorant of the complexities behind constitutional amendments.