Why not take 20% off of all employees salary?
The point is, could you re-recruit on that basis? Can you find enough managers, pilots, engineers etc prepared to take that money? Almost certainly not. BA were having enough trouble recruiting pilots on the current T+Cs last year without a 20% cut.
Can you recruit enough Cabin Crew on that basis? The answer must be yes otherwise BA would be failing to crew LGW flights.