The retail analogy is that if you were a store owner and contemplated the correct number of employees to staff for your store...
In the heavy days of summer and Christmas you need 20 employees in your store. In the lean shoulder months you can get by with 10.
Do you employ 10 year round, run your employees ragged, and allow your customer service to suffer? Do you pay 20 year round and allow your profitability to suffer? Or do you employ 15 and accept that in some months you are overstaffed a bit, and in other months you need to pay out some overtime to staff the operation on the heavy demand days?
The airline would love to get by with 10, ALPA would love 20, and the reality is that we probably are at a point where we have 15.....