Most people opt-out of the doors because either they are engaging their own ID or they find the design not to their liking, especially the D9 series, and D10 is only for Premium flats. Many also wanted PD doors for the bathrooms.
However, more than 70% buyers still choose to opt-in. Probably they like the design, or maybe they just want hassle-free and let HDB take care of everything. Maybe they are also worried that 3 years down the road when the flats are ready, the doors they buy from outside retailer may cost more than what HDB is offering.
Would it be better if owners after taking their keys, look at the interior and decide whether they want to go for the D9/D10 design? How nice if buyers can also choose the qty of doors they need. With current sanitary fittings bundled together with the doors, what should buyers do if they want the fittings but not the doors, or the other way round?
Installing doors are actually quite fast and not too messy. Unlike flooring, you can start moving in stuffs even when the bedroom/bathroom doors are not there yet.
If there are no concerns about cost, quality or availability, does it make sense to install doors only after collecting the keys?
Just some food for thought.