The facts are...
Gf tried to use the BA pilot and Cabin crew computer years ago and it told them to hire 12 more crews to make the L1011 roster work legally.
GF has since 1975 been dependent on crews extending good will,working on rest day,working on days off, and selling their leave.
Sadly "battle field" emergency rostering at GF has nearly always been the way things are done as it is thought to save GF money.
It does however cause a lot of premature mortality amongst crews if done over several years.