Your guess is probably quite accurate for the pax fleet. Nonetheless they're definitely retiring the 744 in stages according to their plan(which everyone knows how "good" are their plans nowadays).
Yes I'm with SQ and yes the number of Capts retiring from this year onwards will be escalating, not exponentially but definitely quite steeply. The background to this is easily traceble if you look at SQ massive intake of pilots(mainly locals) since the 70s. On the other hand, guys are checking out as new Capts too on 777 although I doubt they're able to replace the retired ones. That's why they're going all out to hire cheap labour at this time of the world aviation downturn; both locals and expats.
As for your case, I would suggest you get ready for questions like:
"Are you prepared to work for SIA Cargo ONLY? And NO transfer will be allowed to pax fleet to work with all the pretty stewardesses."
If you say "No" obviously you won't hear another word from them but if you do say "Yes", next question should very well be "WHY?", once you give your part of the bullxxxx( Don't take offence pls, we all did it and will do it again..) they shall immediately drop the next bomb shell..
"Even if your take home pay will be 35-50% less than your peers in pax fleet?"
It's not that difficult to convince them( the typical Singaporean minded interviewers) if you know their mindset, think about it..
Last but not least, don't be despaired if things do get tough in there, just hold your head high in acknowledging your mistakes and stay firm in what you believe is right(Now that'll be tricky at times..). Whatever it is, get in first, work your butt out to get to the right seat and the rest you can decide later; like plan yourself to leave for another airline if they continue to treat you like dirt.