Lifo
In many airlines collective agreements clearly state the LIFO principle as the way to go. In our careers in major companies seniority is held sacred and something that binds us to a certain employer, which is actually a very good thing.
The big dillema companies and unions are facing in a crisis of this magnitude is as said above, cutting of the lower end of the seniority list according to LIFO principles will reduce headcount as per reduced capacity but only just that. Financially speaking it would be like a drop in the ocean. To achieve a substantial cost reduction a huge amount of pilots on the lower end of the seniority list would have to be made redundant. If the top of the seniority list would leave/retire/ be made redundant, a much smaller number of pilots would need to leave to achieve the same cost reduction.
In my opinion possible solutions could be temporary collective sharing of full time employment by working and earning a reduced % similar to the required manpower reduction, another option would be a voluntary redundancy package for the top end of the seniority list, close to retirement which then would (partially) be funded by the rest of the pilot community.