Furlough wasn't no cost though for companies. There were many months where you had to pay at least employers NI (which is not trivial when you are getting little to no revenue), and often a signifcant contribution of the salary too.

There really wasn't an option for many airlines and airports to continue furloughing everyone, they simply didn't have the cash to do so.

Ryanair did have the cash and they didn't lay anyone off to my knowledge - and hence are having way better performance than nearly anyone else in Europe (they also have their own airport ground crews at many airports).