The problem is there are too many 737 F/O's and not enough EMB F/O's. If there are lay-offs then the newer recruits are all on the EMB, the exact pool they do not want to loose.
737 F/O's do not want to move to EMB either because of loss of salary. Same reason they don't want to take up the PB command. Also why would they go to V Aus and have 9 days off a roster instead of 12?