Opposition can literally just converge to the PAP positions over time. Or internal factionalism causes a schism and leads to 2 parties forming from one overwhelming ruling party. In political settings there are enormous incentives to set up roughly 50-50 coalitions.
It seems that Singapore/PAP figured out that policy control could effectively keep power without the violence traditionally associated with authoritarianism. I wonder what other dark arts they employ.
China and Singapore showed democracy is not necessarily the most productive way to run a country