Main point is the current system, PAP holding around 90% of the seats are not ideal. Oppositions' voice just get drowned out as they lack the presence in parliament due to not having seats (1/3) to block bills.
The current FPTP system is obsolete now, most 1st world democratic countries except for US/UK typically have some form of proportional representation system in place.
No system is perfect, PR definitely have some flaws too but it is just that current system we have (With GRCs, etc. and all) that have made it lopsided in favor of the ruling incumbents.
1 party rule has never lead to positive results in the long turn. Cuba/Venezuela/Russia/China are good example of it.