Well you don't want an exhausted flight crew, trust me. Those rules are written in blood.

Yeah, I’ll absolutely take the delay over exhausted crew.

It’s just, the airline never tells you why the flight is delayed, it’s just ‘delay’, and you tend to assume incompetence instead of anything else.

Yes me too. But in a big transit hub like London, with thousands of hotels, and a continuous stream of flight movements, I would expect them to be prepared for such scenarios with relief/standby crew.

Flight crew can't work if they have young kids? Or puppies? or noisy neighbors?

I’m sure the airline can get a much sweeter deal if they show a high volume of rooms booked. Although the costs of an entire days worth of flights being cancelled because of a errant fire alarm seems like a high cost to pay, I’m sure they gamed the numbers out and it makes financial sense for them in the long run

Yeah, they didn’t receive their minimum rest hours or something along those lines.

My suspicion is the crew were up partying and had a smoke break inside which set off an alarm.