I think the guesstimate that JFO time will go to 5 , just doesn't pass the smell test. An increase of 67% just because one more SO (& one less FO) is required on some ULR flights
I suspect your 'smell test' is based on a straight-line calculation.
An S/O joining today has 560 S/Os ahead of him/her. For the first 6 months of this calendar year, there was a whopping total of 33 J/FO upgrades. And a whole lot of direct entry FO recruitment. CX is constrained in growing at around 4.5% per annum until all 3 runways are operational at Hong Kong - year 2030 (source: Chief Exec at crew briefings).
So tell me again how long an SO will be stuck in the back seat?