The upgrade time from SO to JFO is currently about 4½ years. It's projected to go to somewhere near 7 years.
The tranche of pilots resigning are primarily senior FO's with marketable hours. There are some captains leaving too, as well as SO's.
More sectors are being planned with a CN/SFO/SO/SO compliment which is increasing the need for SO's, and decreasing the number of FO's to make up crew numbers.