With any fleet of mechanical objects be it cars, buses, trains or planes that at anyone time there will be issues with one of it.

If these issues were all for the same plane then I'd be worried about that single plane but not with the whole fleet.

I would even say that with a group of 50 staff that at anyone time at least one will be off sick with some ailment but it doesn't mean the whole team is ready to fall off this mortal coil.