
SN does not have a large long haul fleet, so they can do two things, either run the schedule with "all" aircraft operational (save for scheduled maintenance tasks and a little slack built in through timings etc.) or keep one aircraft on standby in BRU in case something goes wrong.
The first solution means delays will be compounded, and it might take days or even weeks before the ripple effects from a disruption in the schedule have been gone completely, especially if you have more than one aircraft with problems at the same time.
The second solution might sound like a better option for the passengers, but it would cost them a heck of a lot of money, which wouldn't really bode well for the continued existence of the airline.
With a small-ish fleet, the first solution really is the best, even though you might inconvenience some passengers at some times. It still beats going bankrupt...