Maybe SMRT buses are not like SBST buses, Their lifespan is
shorter? That why Mercs got extra so that SMRT can find buses to
replace
I notice most of the SMRT trunk services cover very long
distances, unlike those covered by SBS. Their break time for each
trip is also considerably slower. For example, svc 65 covers around
25 km while svc 963 covers around 30 km. If you observe at
Habourfront Int, when 1 65 departs, a couple of 963 buses have
already come and gone.
Imagine how much mileage each bus cover daily, and how hot the
engine is at the end of the day. With their standard of
maintenance, it is no surprise that their buses don't last long.
The O405 are like an exception, but again there are a handful of
O405s that are in bad condition.